I want to try Mtn biking again with a bike that fits me. I borrowed one for my first trip and the two things combined left me with a TON of bruises! I loved it though!!

I did a road ride last night (first one) and got basically dropped on the first hill - very frustrating!! How long does it take to build strength on the hills?? I was riding at about 20 mph on the flats and feeling strong and good but as soon as theres a hill I bonk!