Cycling. If you have a bike you can choose how much power you put through the pedals. I used to commute everyday by bike and I'd really put an effort in each way riding as hard and fast as I could. And one flat I lived in was at the tip of a brutal climb which I used to attack every day, arriving back spent. As it happened I did that same climb today and I struggled to get up it at all, I was definitely fitter 10 years ago!

I took a break from riding for a few years and barely touched my bikes but am now getting back in to it. But this time I'm deliberately trying to go slow. Find the local trails and low traffic areas and pootle along. I've been having great fun just exploring. But then if I want to try ride up the big hills I can do that too. I also never used to stop as side for traffic. But you can stop whenever you feel like. Bring some food and a drink and stop for a few minutes or at a cafe.