Just wanna echo what other people have said and add my own two cents.
Some of it is just getting used to it. When I'm feeling lazy, it is easy to let my pace drop ridiculously. That's rather infrequent, though. In spite of momentous strength gains this year (admittedly mostly upper body), I've not substantially changed my pacing because I haven't fiddled around with my cadence and gearing.
Oh, yeah, I use music to set a cadence whenever possible. Since almost everything I listen to is EDM at around 128 beats-per-minute, it means I've got a cadence so predictable that I literally do pedal in my sleep. I should probably try to find something listenable at around 140 BPM. Your own mileage may vary, but I find that having dialed in my comfort speed, I forget to boost it unless a really good song comes on, I am racing a red light, or someone just made me angry.
Lifting heavy things (or even just doing good old bodyweight squats) will help with strength. Question is whether you need the strength. If you are 50 lbs. overweight, that means you're hauling around 50 extra lbs. Heavy people who are active tend to be in at least as good a shape muscle-wise as slimmer folks, sometimes better. If you want to be stronger, definitely do some lifting in between the cardio of biking. I could write up a fairly simple bodyweight routine to start on if you're interested.
Also, and I'm kinda surprised no one mentioned it, your bike has a lot to do with your top speed. I ride a hybrid since it is my commuting cycle. If I were on a "road" bike (a racing bike, properly), my speed would jump by a rather significant interval. If I was after pure speed, I can comfortably say that my bike is holding me back.
| current weight: 199.6