How much Pool Running and Cycling to maintain fitness if I was doing like 70-80mpw?

There is not enough time in the day to do enough to equal that many miles running.

^ he's right, you could up the intensity though?

Lots.

You should check out some of the workouts LR poster Darkwing did during injury rehab.

I'd also add swimming to the water portion of your workouts.

What type of injury or other situation is behind your question?

When I was injured this summer I took up a bike for a month. I did as much as my knee allowed per day. Sometimes getting to 40 miles. When I was able to run again it felt like I had the fitness of a 9 year old child.

No time doing anything else beside running will maintain your running fitness. I assume youre injured, go do the pool running and cycling if you want, it will keep you sane, but dont expect anything out of it.

Dr. Tobaggan wrote:

go do the pool running and cycling if you want, it will keep you sane, but dont expect anything out of it.

Tell the late Bill McChesney that.

Water running will not replace regular running, but if you do interval/tempo type workouts, the degree of lost fitness will lessen.

20-25 hours on a bike or 400 miles a week will help you keep your fitness. you may lose some of your running fitness but other parts of your legs will get strong with cycling