Hi (new member so before I ask for help - thought I would volunteer some)
Most "mature" triathletes come from a running background - myself included. Younger triathletes can come from any background most noticeably swimming.
The mistake most runners make when they try to ride a bike is to use too big a gear.
As a general rule of thumb, if your quads hurt, you are in too big a gear and you need to down gear. (Make is easier to pedal)
If your heart and lungs give way before your legs - you need a bigger gear - harder to pedal.
The secret with triathlon cycling is to try and conserve as much energy for the running.
(Hence you see tri bars to lower wind resistance amongst other things)
Newer cyclists should stick to the smallest chainring (on the front) or middle if it is a mountain bike and the sprocket on the back with the most teeth. (smallest gear)
Pedal as if you constantly need more resistance and it feels like it should be harder to pedal and you are probably in the right gear.
And because your saddle height is probably way off, you won't risk damaging your knees (but that's another post...)
You MUST ALWAYS WEAR A HELMET and try to use as hard sole shoes as possible and think about moving up to proper cycling shoes and pedals as soon as possible. Mountain bike shoes and pedals (generically called "spd's" although that is a brand name) are probably the first step
Modify your stretching routine (you do stretch don't you...) - especially in the first few months of cycling because your running muscles are now being asked to do something very different..
Hope this helps you - I learnt all this the hard way
