Now that I've taken off running a little bit I've put on a fair amount of muscle.
If you have a house, do a lot of gardening and odd jobs like that to keep you on your feet on use your muscles; it made me bigger.
In college, I used the starting strength routine. It is an excellent thing to start out with. Your legs are likely much stronger than you know. I went from Squatting 175 for sets of 8-12 to sets of 5 at 255 within the span of a month (I did work out more than most runners though). My deadlift max was about 300 pounds. As soon as you get reasonably strong (the definition for the changes, but once you start stalling out more often and feel that you have been able to build a strong core) in those lifts I would move on to some more upper body focus.
There's a zillion articles on how to train upper body. Once your lower body is strong enough, you can really do anything. But try to stick to compound movements.
Finally, just eat a lot. Eating the right macronutrients gets more and more important the bigger you get. For 3 months you'll be putting on muscle like its nothing, but then it will get more difficult. There's also a zillion articles on that. They're mostly correct.
Good luck.