Single Speed or Road

As an MTB'er I normally have a problem with my heart.. ie, it beats to fast and I feel like dying, recently though, I was out training on the road with some friends (on my MTB) and found that my heart was fine, and it was just the muscle(s) in my legs that were giving out on me.
So I was told the best thing to do, is higher mileage with fast spinning, so would it be better to get a single speed (ie Trek1) or a normal road bike.
Apart from a little amount of road work, and nipping through the city, I only go MTB
Cheers in Advance
So I was told the best thing to do, is higher mileage with fast spinning, so would it be better to get a single speed (ie Trek1) or a normal road bike.
Apart from a little amount of road work, and nipping through the city, I only go MTB
Cheers in Advance
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Road riding does seem to give a more constant steady effort, where as off road the intensity varies more.