It is not the speed per se. I can easily run 10 mph on the treadmill for a short period of time. Or even 9.0 mph for half an hour. But anything longer than that becomes very difficult. I suspect the cooling is to blame and eventually the lack of efficient cooling gets to me.
On Wednesday I took the little ones to the pool and swam a little over 2 miles (or 180 laps). I swam about 1/3 of that with a pull-buoy to give my arms a workout. I have noticed quite a few times now that I can swim faster using a pull-buoy and no leg movement whatsoever.
Yesterday Alistair and I drove to Walnut Creek and rode up Diablo. It was very hot (93F at the base) and I suffered on the way up. The time-trial course (to the halfway point) took me 35 minutes but I needed another 10 or so to recover. Alistair was quite a bit faster, but even he complained of the suffocating heat. Not so good. After "recovery" we rode up to Juniper but at a slower pace. Then we turned around and came down. Even though this is only a 22 mile ride, I was surprised to see that I burned well over 1,500 calories doing it.
Today I rode 35 miles. Out over Wildcat to Orinda, then Moraga and back over Pinehurst. It went pretty well and I rode hard.