Hi mate,
I can only relay what the guys have already said -
You can gain the extra weight without steroids as it seems u primarily trained for endurance and fitness?
A good 3 day routine with plenty of compound movements and a good mrp/weight gainer will have you well on the way to bulking up naturally - get plenty of rest also.
Try and eat every 2-3hrs to keep a positive nitrogen balance and drink 3-4litres of water a day to keep u hydrated.
Follow those simple enough rules and u will gain well without gear
