Banjo, I don't know your starting point but you may be surprised at the speed of improvement.
I started cycling to the station in Nov08, round trip of 7 miles. I'm 42 years old and was very unfit and 18 stone in weight. First few times I couldn't complete without a couple of walking spells. I'm now doing my full commute, round trip of 40 miles, 3 times a week. I also had most of Feb off the bike due to snow and illness. I've got a triple hybrid and without the granny ring would never make it up some hills. Some stuff I've learned:-
saddle height - I'd gone with the old idea of being able to touch toes to ground both sides simultaneously. Now I've got the height right I can only touch with one foot at an angle. It's actually easier to slide forward off the seat and stand over top tube.
hills - I'm getting better and find if I can stay in the saddle I get up them with less effort.
gears - initially I ground out too big a gear and felt some pain in the knees, I spin a lower gear more now and it's much more comfortable. My legs do ache but not in a bad way and I'm resting in between commutes as well. My gear range, in inches via Sheldons gear calculator, is 31.4 to 117.7. I actually have only been in the big ring once to see if it worked. The top gear in the middle ring, 95.1 inches, seems to be plenty for my needs. I am using less at the other end and haven't needed the 31.4 this week and only once used the next gear up. It has made me wonder about gears and so I've got a new wheel with a 9 speed hub gear. I figured I only used about 7 of my, alleged, 27 gears anyway.
improvements - I've lost 21lbs in weight, I've gone from an average of just under 10mph to 12.96mph(today) for my 40 miles. I no longer feel like I'm about to die at the end of the ride.
Beware - this cycling thing seems quite addictive, I've certainly been bitten by the bug. I resent my train commutes but do know that too much too soon could do me damage