How long before a 25 minute 5K?

Too many unknown factors. Age, weight, undisclosed health issues, etc. At age 45 I went from high 28s to sub 25 in less than 4 months and I wasn't logging as much mileage as you.

You have a good start, but you don't indicate you ever run harder efforts. I'm a big proponent of run lots, mostly easy, sometimes hard. It seems like you have the the first two down, but you need the third item as well. Maybe just adding some striders to the end of your runs will be helpful.

Plus if you have weight to lose, then do so. It's much easier to go faster without carrying excess weight.

Age=55

6'3" and 180 so no weight to loose.

Did nothing my entire life until age 45 when I started mountain biking. It took me three years to get where I am now but I have stayed the same from then.

I figure in a couple of weeks I will start one tempo run a week and after a couple more weeks I’ll do some striders at the end of my short runs.

I am curious if you are doing any speed work, hill repeats or whatever to build up speed or just running steady all the time?

Since 5km is so short speed work is essential to go faster. For a 5km you're going to be running pretty much at your lactate threshold the whole time. So do speed work at and above your LT and you'll improve your LT therefore increasing your time. Doing more mpw at a slow pace will allow you to go farther, not faster.

Interval training or hill repeats is the way you will get faster.

One way is to do a 3 mile run at a high school track:

Do 12 400's the first time out--as fast as you can with rests for the amount of time it took you to do the lap before starting the next one);

the next week do 6 800's;

the next week do 4 1200's;

repeat. 

I would add that it's some very well respected people would say that.

Keep running.  At that pace and mileage you will get faster by simply continuing to do more. I don't know how long you've been at 24 miles, but you may be able to add in some easier speed things. I don't think it's that necessary yet.

What you can do are things like strides. I'm sure someone else can help give better definitions, but there are a few ways to do them.  Late in a run, increase pace for 30 sec or 100m.  Go faster right away, or gradually build over the period to say, new estimate for 5k pace.  Repeat just a few times at most. See how it goes and do that in maybe a couple runs out of the week.

You can also use the hills. Go a bit faster down them, let gravity help carry you. Get your legs used to working faster, but (this is important) make sure you are still landing well. You could push yourself up the hills as well.

Long slow distance (lsd) is not just running the exact same pace time and time again. That's how people get stuck at only running one speed. Seemingly little variations like above help to break out of a rut. The lsd is still very important for building volume for later work.

You have been running enough to add in strides now. Even 10-15" at 5k 'effort'. Eventually you can extend to 20-30". This one workout will help changes quite a bit for you. 

Tempo running: one of the under valued aspects of getting faster. Add in 10-15' of tempo 1x per week. Sometimes make it hilly, other times make it flat. Mix it up. 

Long run? Not sure you mentioned this, but if you are 1 hour or more, that's plenty. 

Let us know how you improve!

For anything longer than 5km maybe, but for such a short distance you're not going to be utilizing your aerobic engine much, you're going to want to be near LT to maximize your potential and get a good time. The way to increase that is with with say 3-15min tempo pace intervals (your ideal 5km pace) mixed in with easy running before, during the rests, and after. So you're still getting your easy miles in but incorporating some speed in there as well.

High volume, low intensity will work. It's not as affective as you're building a very strong aerobic engine, but not doing much for your LT or Vo2 levels. Where in high intensity, lower volume, your aerobic engine is still getting worked on (although not as much) but you're increasing and getting your body used to going at or near your LT pace.

I'm no expert on the matter, just done a bit of reading.

Missed that, I sat on the post reply thing for awhile when my boss came in my office to bs, and got in the way of me replying.

You may have some potential. How often do you race?

I'm in your age group and will have done over 30 races this year. I've been doing something similar almost from the time I started 11 years ago. Because I race so much I know the people who are faster than me. Over the years I've worked to be able to beat these guys. I also use other people as pacers. If I know that a certain person normally runs a certain distance in the time I'd like to achieve, and I'm realistically within range - I'll run just behind that person for as long as I can. Trying to keep up with a slightly faster person will pull me out of a comfort zone. Getting out of the comfort zone is crucial when trying to run a faster time. In focusing on trying to keep up, some of the pain seems to lessen.

One other thing about racing so often against the same guys is we've all become friends. We socialize before and after races and encourage each other.

I encourage you to keep running or even increasing from the mileage you've been doing. You need to add in a small percentage of faster/harder efforts and you should start seeing better results. I will say that when I started averaging 40+ miles/week regularly my ability to hold a faster pace in races greatly increased even though my training pace didn't. When I bumped that up to 60+ I got even faster and even though I'm not running that much this year I've been able to hang on to most of the increase and set several new PRs at age 55.

Have a long term goal too. I pushed myself quite a bit in my early 50s with a goal of being consistantly top 3 in AG when I got into the 55-59 AG. Keep up working and pushing yourself for when you join the 60+ group.

That's all I've got for now.

Running distances longer than ~800m (or ~2 minutes) are not terribly different from one another in training methods.  The aerobic part plays a significant role in anything longer than this. The amount of intensity depends on how much one has built up to be able to consistently handle it. Mikericci gave a nice response on building into this for the level of running the OP is at.

You ought to start considering doing some speedwork AT LEAST once a week. I like Endurance Nation's motto "Fast then Far." So the offseason is a good time to work on speed, then when things warm up in the spring, you can start stretching that speed into some longer runs. 

Over the last year or so, I have been training under the wing of an elite runner. This link will show the you the VDOT charts that we use when we do speedwork. http://fastforwardtriathlon.com/wp-content/uploads/2010/08/VDOT_Chart_from_Daniels_Running_Formula.pdf

If you have never seen this before, what you do is find your most recent 5k time, which I believe you said was about 30min, which equates to a VDOT of 30-32. So then you use that VDOT to find how fast you ought to be running your speed work. Feel free to PM me if you have an questions. 

It sounds like you are a healthy individual, I think with proper training you could be at 25 mins by perhaps the middle of summer.

A 5K is absolutely an aerobic race. 

To the OP, hard to say how long it would take, but I would encourage you to race a lot (every 3-6 weeks) and don't be afraid to go for it and blow up, even if it means you end up walking a bit.  I think part of the issue might be not knowing how to race.  I have raced a dozen or so 5Ks and still don't really know how best to pace myself.  The week before the race I will do 1 interval type workout to get an idea of where my fitness is, but its a 5K.  If I blow up, I can find another one relatively quickly.  You will get faster by staying consistent with your running, mostly easy as others have mentioned.  Your running economy will improve and you will race faster by racing more as well.

There is some really good advice here. Strides are probably the best way to get started on improving your pace.

At 55 years old you will have to worry about injury. I am not the fastest runner but have made steady progress with consistent long steady runs. It has also helped keep me from developing a lot of the nagging injuries that are the result of intense speed work. Be patient and  put in the miles, and you will hit you goals or better.

How do you feel during and especially as you approach the finish line of a 5k? If it doesn't hurt like hell you're not racing hard enough.

I would also be careful.  I am only a couple of years younger but this year working on running a little faster I tore a calf muscle.   This was only 2 weeks before my IM race .   

I would make sure you also listen to your body in your quest.