Newbie needing help

Hey guys, I recently started bike riding again, and I've forgotten how much fun it can be! So now I'm determined to get better at it, and want to have a concrete training plan that I can stick to. And I'm in need of some advice.
I am 5 foot 10, and only 125 pounds, and pretty weak to be honest. So should I start by gaining some weight and building leg muscles? Besides riding, are there other exercises that I should be doing?
Also, just out of curiosity, how fast do people usually go? I notice that on my daily route, I usually ride at around 15-17 miles per hour (mostly flat with gental slopes periodically).
Thanks a lot!

 

How many times, hours a week do you spend riding? The more you ride the stronger you will get.

 

Don't bother trying to build leg muscles. It won't make you faster on the bike. Once you get faster and maybe consider racing you will be trying to lose all that weight anyway. Be happy that you're very light and enjoy riding your bike.