Careful venturing too close to cycling as it will pull you in, drain your bank account, break your collar bone and make you a slower runner. Sure, you were only going to do a few easy miles a week on a borrowed mountain bike, but if you run quite a lot, you'll probably be quite light, have decent aerobic capacity and find that for a beginner you are pretty good on that bike. A bit of encouragement and you'll start the progression to spending all your spare hours riding your megabucks cervelo with deep dish wheels, doing the duathlon math (if i gain x seconds a mile on the run how much of a cushion do i need on the bike to be the best duathlete in my town). You may look cooler, but i doubt you'll be a faster runner and you'll have to be careful not to overtrain.
There was an interesting article in bicycling magazine that mentioned the difference in power application between the 2 sports. in cycling the power is applied almost uniformly and constantly, in running it is that split second of contact time each stride. i think that may explain why runners tend to slow more with age than cyclists.
i started as a runner and have done both with varying intensities since about 2006. what is nice about cycling is that it is something i can almost always do when i have running injuries; and, if you decide to take it a bit more seriously (after you have been running for a lot of years) it is great to feel the improvement you'll get in the first few years when running has stalled or is going backwards.