Sean, what you are proposing is what I mostly did for 6 months this winter. I’m home early to babysit, in return for time off at weekends, so long evenings at the wall are unusual now. My sessions typically consist of a brisk walk to the wall ("warm-up"?), 10 minutes moderate traversing or circuits, 5 minutes rest, then get properly stuck into problems for strength/power or, latterly, anaerobic endurance for 30 mins or so. I use a watch and have set resting times to make sure I do a decent volume. 25-30 problems at about 80% max ‘on the minute' feels like a decent anaerobic work-out, as does one 4x4 or 4x6 (2.5 minutes at 80%, followed by 2.5 mins rest, repeat 4-6 times).
One of many gems in ‘9 out of 10 cats’ is that doing a very little training is far better than nothing. I'd always try to do something unless I felt completely crap. If it was a 'high gravity' day, I might just do 20-30 minutes of easy aerobic work.
I tried to have a snack of some kind immediately after and a 'good rest' at my desk or on the train home!
Result was 6 months good training and no injuries - previously unheard of - and consequently feeling stronger and climbing better than I have done in 10 years. Ironically I hurt myself, not badly I hope, after stopping for 3 weeks.
I am old and untrained so can't cope with much volume on plastic but I'd guess the same principles would apply until you get to fairly serious grades or were aiming for long pitches and long days and needed to train in higher quantity.
All this relied easy access to a wall and, as Serpico suggests, an unlimited climbing deal. It's hard to justify £10 for a 45 mins visit.
