When I can, I generally run both A and B, with a 5 minute pit brake between sessions. I'll throw in a few C sessions later in the day to spend time with my wife. Yes, that means 40 minutes on track, with 20 breaks, and the occasional 80+ minute stint (with short breaks.)
I don't do any special training and conditioning, although subjectively I seem to have somewhat above average endurance for someone who doesn't train regularly.
The most valuable bit of advance I can offer is that relaxation on track is way way more important than building endurance, and endurance is way more important than building strength.
I'd personally recommend bicycling, since it builds leg endurance and is relatively low impact on your knees. It can also help build some wrist endurance.
A lot of problems can be solved with technique. Making sure you're shifted off the bike before breaking or turn entry will significantly reduce energy needs during track riding. Relaxing your legs and arms during cornering will not only reduce strain, but will also improve feedback and handling from the bike.