Lungeing is good, if you have a sensible horse. You can't beat riding though. I agree with what most of flamepony12 said, and about the muck heap, which i don't think she mentioned, you can do different things with it...

1. Burn it. I know some people that do this, but fire is dangerous and you'll get a lot of smoke.
2. Sell it! Menure is the perfect fertiliser!
3. Keep it in a small pile and take it regularly to the tip. This takes a lot of trips back and forth, but it's probably the best way. Menure piles are annoying and not everyone wants one near their house