Haven't been to a proper gym since I left uni in 2000, but what I did like using was the rowing machines - top notch for all round cardiovascular fitness, no impact on injured ankles and kness, along with being excellent for rehabilitating mid-level to serious ankle injuries.
Started running a few years back when I broke my wrist, couldn't skate, ride my bike (no strength to pull the brakes) or do yoga, with the result that my fitness just plummeted. Obviously enough, the morning run has really helped my fitness in general but also vastly increases stamina & endurance with skating which lets me skate longer, just have to make sure to stretch loads before and after each run, take a cold shower as it helps with minimising cramp & stiffness and go for a short walk afterward/before skating to loosen up a little. If it's pissing wet, I'll do circuit training at home with some weights, I've found the Spartacus workout pretty good. I'll go for a run one morning, yoga the next, training/running the third day, yoga on the fourth and so on - gotta start every morning with a vigourous workout or else I'm slow and useless for the entire day.
Yoga's badass, been doing that for over 10 years, highly recommend it for overall waking up your body and stretching out all the muscles that get no attention & those you need for skating - all skaters will benefit from doing the cobbler/bound ankle pose, hip opener pose, seated & standing forward bend, wide legged forward bend (great for the inner thigh & groin area), kneeling shine stretch (also works the ankles) spinal twists & wheel pose. Look up power & ashtange yoga on Youtube and you'll find tons of relevant classes, routines & breathing exercises , anything from 30-45 minutes every day or two will do it for you - if you do any martial arts, a lot of your warm-up stretches will be the same.