If you don't overdive and there isn't too much oversteer (which involves a higher work rate) then outdoor karts are pretty sedate physically. I'd never claim o be fit but I can walk away from a 115 minute stint (maximum range of a fuel tank) and don't feel tired.
The track we are going too isnt too bad on work rate, it's a little bit twisty because they only run single engined karts but it's far less work than a typical indoor track.
Np on the lift, I guess I'll be doing the whole London thing afterall. Grr.