Sapper - just keep riding. Do what you can, and then add a bit more. Runners do the 'add 10%' seems a good one.
If commuting, ride 1 day, week 2 ride 1 day, week 3 ride 2 days..... and slowly increase.
It's a long road. GB155 is the best example of bloody hard work on here that you will find, and he can advise you - I can't...
Me I was a racing snake until late 20's - job change/kids got only 2/3 stone heavy, but my, was it hard work getting back after 10 years not commuting by bike. Doddle now ! So keep at it. It takes a couple of years.
Don't get down, you will get to the point when it click's.. I battled to get fit again, and there was a commute when it all came to.... then you just build on it.
PS hills are always hard, they never get easier. You just get faster.