One of my ongoing 101 in 1001 challenges is cycling to Mordor, a fitness challenge originally called The Éowyn challenge or Walk to Rivendell.
I hate most fitness challenges because they are either about losing weight/getting smaller, or they quickly become impossible for someone who is not already pretty fit (yours truly). Never mind that they seem motivated by some kind of public humiliation and belief that health is some kind of moral responsibility. (It isn't, by the way. Neither is happiness.)
But walking (or running, or swimming, or cycling) the Lord of the Rings novels just takes mileage. It doesn't say anything about how long it has to take you, or how you have to do it. You just keep adding your numbers on. That's all there is to it. If you want, you can make yourself a map to keep track of your progress. I did, because it's really neat to "see" yourself travel through Middle Earth:
I began in around October of 2013, and so far I'm nearly to Rauros Falls. That's like 2127 kilometers. (My brain is used to miles but my stationary bike is in kilometers so...) Is that the snappiest pace? No, obviously the Fellowship did not take eighteen months just to get to the breaking point at the end of Fellowship/the beginning of The Two Towers. But 1) they had horses and 2) they're fictional. The point is, it makes hopping on the bike and sweating for a half hour just a little more rewarding, and I'll take all the reward I can get because I like moving about as much as a sloth.
If you have a hard time getting excited about movement, this might be just the thing to get you moving. (If you're a Tolkien fan, anyway.) Enjoy!