Assuming the bike is in decent nick, as in - not rusting etc, and you've had someone who knows what they are doing give it a look-over, then if you plan on sticking to the main fire-roads etc., there should be no problems - your main consideration would be that the brakes are in good working order, since you could build up some steam coming down... If you want to hit the singletrack with it, well then again - that would mostly depend on the condition of the bike... Regardless, a fair amount of common sense, and provided you don't go and throw yourself off major drop-offs ect., you should be fine... My suggestion would be that you first start getting into the cycling again, hit the main and basic trails first, and you will quickly get a sense of what the bike (or you, for that matter ) are capable of... Should the bug bite - you can then start looking at upgrading...
