Help me with my post race analysis

I had my first swing at the Olympic distance last weekend and had great fun at Wasaga Beach in Ontario.

I do need some help analyzing what went wrong as I found the last 5k of the ride intensely painful and jogged most of the run with terrible fatigue.

The two weeks before I was on vacation and able to go riding with plenty of hills. I did two rides of about 35km each with over 1000 ft of climbing. I really enjoyed them and my felt I would boss the ride. I ran four treadmill session each about 30 minutes, three at a tempo pace on the cross country setting for some variety and the last 3 days before the race at a slow pace. I felt I had great legs for a 10k. I swam in the lake 6 days before the race to check my sighting options and get a feel for the beach run up.

The swim went great. I was not fast but swam within myself.

Nutrition. I made my own mix with Gatorade crystals mixed quite strong with a vega sport electrolytes portion added. This has done me well despite being quite thick. I had a bottle to gulp in transition, a full bottle on the bike and some in a run belt. But come the run I was gasping for water.

Last 5k of the ride I started to ache terribly. I am fine in the saddle for 3.5 hours (Paris to Ancaster x-cross race this year with no saddle pain) but my glutes and quads started to kill me. Starting the run my quads were tight and felt I had zero power.

Theories:

Too much ride climbing in the last two weeks left some fatigue i.e. Poor taper

Poor nutrition and hydration - single bottle of intense mix on the bike not enough fluids

Just not fit enough - a distinct possibility. Maybe I don't realize what training I should have put in the bank but I felt well prepared

Mystery problem - I have not been sick but who knows

Anyway. I have a bag of theories to test out to try and do better next time. Anything stand out to you good people as an area to improve?

It's none of the things you suspect.  Primarily, you biked too hard.  Secondarily, during a taper the majority of your training should be at your planned race intensity.  Neurological adaptations occur quickly.  Training right before a race at lower intensities reprograms your nervous system to fire muscles at that rate and intensity, leaving you flat on race day.

Dehydrated?  Maybe.  Caloric slump?  Unlikely.  240cal (assuming mostly carbs) should be fine for most in an oly.  You may be able to go a little higher, but don't start thinking that you need to replace 1500cal during the race.  All you need is enough carbs to keep your engine burning.  If your drink mix is very thick, you should always be drinking water at the same time to dilute it in your gut and make it easier to process.  Same as if you used gels or solids.

Dehydrated?  Maybe.  Caloric slump?  Unlikely.  240cal (assuming mostly carbs) should be fine for most in an oly.  You may be able to go a little higher, but don't start thinking that you need to replace 1500cal during the race.  All you need is enough carbs to keep your engine burning.  If your drink mix is very thick, you should always be drinking water at the same time to dilute it in your gut and make it easier to process.  Same as if you used gels or solids.

I think the take aways must be:

Match training intensity to race conditions. Doing a bundle of surprise climbing during taper when I needed to make a steady flatish TT effort was a mistake. Throwing in some treadmill hill climbing efforts before a pancake flat 10k is daft.

Nutrition. A basic calorie calculator estimates the swim at 602 cals. Strava lists the bike effort at 888kJ. A shade off 1500 calories and I calculate that all I had on the bike was about 240 cals in my drink mix. Combined with a sunny day at about 27degrees I think I ended up on the dehydrated side and in a calorie slump.

Biked too hard for my ability. Looks like I should start saving for a HR monitor, power meter or some bike tech so I can measure and control my effort.