FWIW, I turn 30 in eleven days.

WARNING! Odd mix of metric and imperial measures!

I'm 6'0", 237lbs. (eg: 50lbs+ over where I should be)
Three months ago, I was 260lbs, averaging 10-12km/h and had trouble doing 5km.

Yesterday, I did 57km over the course of 3.5hour averaging about 17km/h.
I was VERY pleased with that. I'm actually less wrecked today than I was two months ago trying to do half that distance.

It's all about conditioning. (I bike every day & will be going to spinning classes when the weather gets cold)