Concerns with 1/2 IM plan

That is the problem with generic plans .... they are not tailored to your specific needs.

My suggestion is if you are comfortable and recover well then it is fine to add some distance to the longer days.

Just remember, it is not one single workout that makes a plan effective, it is the total combined load of the plan.  Even if you only ran an hour a day each day 5 days a week, you are still looking at 5 hours of running a week ..... which is a decent amount!

If you need to run a full 13.1 for your psyche, is up to you.  But if you are following your plan, it's ok that you only can get up to 10, IMO.  You WILL be able to grind out those last 3 miles in the race.

For example, most marathon plans have you max out at 20 miles a couple times before the actual race.

My IM plan had me out for 3 hour long runs.  I could only do 17-18 miles in that time frame.  I was able to finish just fine and turned in a pretty good marathon split (sub 5 hours) considering it was my first IM and first marathon (first time running over 18 miles, actually).

I wouldn't sweat it and stick to the plan.

If you make any changes to the plan, I would recommend that you start by trying to build more run frequency rather than be overly concerned with your long run distance.

And, since running is your weakness, I would implement the more running before adding onto the bike rides.  If you can do the former, then feel free to add the latter.

I used the silver 20 week HIM plan last summer for my first.  I did a half mary but I trained for it over the winter months and did it in the first week of April and was only 6 or 7 weeks into the HIM plan.

Prior to my HIM in mid July I did plenty of runs in the 1 hour 20 min to 1 hour 30 min range for my long runs.  I didn't find a need to punish my self with longer runs with all the other training I was doing.

I'd be real hesitant to be throwing in a half mary program during your HIM program with everything else going on.  If you like to add time to your training do it in the bike or swim, pace yourself on race day and you'll do fine on the run with long runs of 90 minutes that don't beat the crap out of you.

JMHO, good luck.