cycle more, take it easier, try not to go into the red for extended periods to let your body adapt, then gradually extend rides by about 15km a week. I've a tendency to try and tear my legs off every time I go out but if you're in oxygen deficit all the time it's not helping anything, so it's better to ride to your heart rate monitor and stay within zone 3/4, increasing the speed as you get fitter and your heart rate drops.
Such internet advice. Well meaning I don't doubt but... Booo.
http://cyclingtips.com.au/2012/08/starting-from-scratch/
this is what i meant. Don't fink it's so stupid.