Please don't over do it!
I dug a new bed today, it wasn't that big. But every year I'm shocked at how out of shape I get during the winter. I mean, I tried to lift weights and everything! I got out there and just about died after 2 hours. By the end of the gardening season, though, I can go for 4 hours easy. :)
When starting out, please remember to stretch, drink lots of water and take frequent breaks even if it is cool out. Please, please, do not bend down so that your head and shoulders are below your waist. Squat, resting your bottom on your heels if you can. If you can't, kneel on a pad or pillow. No one wants to start spring with a slipped disc.
Please use tools correctly. :) Just a glance at a how-to site can save a lot of sore muscles and injuries.
Have fun out there!