shedfull - very insightful - I too suffer from prolonged calf and achilles problems which podiatrists and physios keep trying to treat in various ways. i was fine running a hakf marathon in 'later life' but suspect my subsequent problems are as much MTB related as that's the newest activity to me over the last couple of years. I've oft said this to physio/pod's that I feel it's cycling that's leading to running soreness thru some sortof relation. Interesting to hear your thoughts on this too !
Cheers.
[i]My question is around what will help increase my fitness most effectively..[/i]
Work harder.
firestarter - I'd guess rower, less impact than running.
Rowers aren't light though, you may burn more fat, but you'll put on more muscle bulk.
This is just my experience, but I've been through a couple of periods of quite extensive (several months duration), consistent and regular (at least 4 times a week) training - first on an indoor rower, and more recently running.
In both cases I got a lot better at the sport in question - but failed utterly to get any quicker on the bike.
I think the only likely crossover benefit you'd get from one sport to another is if you're very unfit to start with, or if the extra exercise helps you to lose weight.
Cheers chaps