Gardening with Arthritis - Any tips???

I was wondering if any of you have tips on gardening with arthritis. I am in my mid forties and have had osteoarthritis in my knees and spine for almost twenty years. I used to be able to work all day in my garden and still be productive without all the aches and pains I get now just after a couple of hours of gardening. So, I am seeking suggestions on keeping active while still being productive outdoors. I made the mistake of overdoing it by working all day only to hardly be able to walk for the next week a few months ago. Then I had to be on strong prescription anti-inflammatories for a month and I felt so tired. The thing with arthritis, at least for me, is that once you get warmed up you feel like you can take on the world until your body cools down later and you find yourself hardly able to function. I was thinking that maybe an hour a day might work better. I have a desk job and work eight hours or more per day so it is tricky getting time to garden in the evening sometimes, but I must keep going. I welcome your thoughts and suggestions.