2-3 days of cross training is generally recommended for better running performance. I have a reclining stationary bike, which I try to use 45-60 minutes, 2-3 days a week. But I have the psychological feeling that this aggravates my poor leg muscles that are already tired from running.
I prefer swimming to biking, as a cross training workout. But, I am a cheapskate and would rather not pay for swimming lessons or costly gym fees. So, when our community opened up our free swimming pool over Memorial Day weekend, I was one of the first people to jump in, dragging DH behind me! Ignore the fact that I could only swim a few feet at a time; pay no heed to the fact that I am incapable of swimming any style and my only aquatic accomplishment thus far was to lie on my back like a pancake, in the water.
I had a lofty goal, though...learn to swim by the end of this summer...so, I plodded along, a few steps at a time, trying to copycat other swimmers and stealing DH's moves (he was also, learning, by the way, so it was blind leading the blind and all that good stuff).
Anyhoo, this satisfied my cross training goal, which was to thrash around enough to feel the tension in my arms and back, every few days. I was surprised by how quickly I was progressing...albeit in an imperfect way. The kids, with the help of formal swimming lessons, were all moving swan-like in the water, while I was darting from one end of the pool to the other like a very inept tadpole, with the aim of just getting to the other end. I swam industriously 3-4 days a week though, and within a month and a half and the help of DH and a neighbor, Ulla, the tadpole became a frog! I can now manage a clumsy version of breast stroke and a tardy, tentative back stroke, besides a very breathless crawl. DH has not been as regular as me, and he was quite impressed when I exhibited my technique yesterday, as well as completed a 1/2 mile in laps. Man, I was pleased as punch! Onward to the butterfly!