I'm of the 'carry more line' school. Carry what you can control then shoot the rest.
Agreed, 100%. The only way to extreme distance is a forward cast off a very good backcast - one that straightens out completely with very, very little sag. And the tracking HAS to be dead on, as well as the turn around timing.
In order to get extreme distance with the MED you will need to be carrying at least 70' of line as Jackster said. If your rod isn't powerful enough to accelerate that length of carry (many aren't), downline your rod. One of the top tournament distance casters in Europe says an 87' carry is optimum for him with the 5 wt MED. He used to cast the 5 wt TCR and I think is using the 5 wt tcx now. Another has shortened his carry somewhat but it still approaches 80' I believe. The differences are not only in the rod, but in stroke length and haul speed.
You are going to have to play with your casting style to achieve that kind of distance. Nearly all successful tournament distance casters use a vertical casting plane, and a wedge loop, with the fly leg higher than the rod leg. But the casting style varies significantly. Some arch their backs incredibly on the backcast using an open stance, bringing the casting wrist through the space just occupied by their heads. Others, often the tall guys who are very strong, use a closed stance, right foot forward (for right handers), pushing off with the right foot for the backcast and keeping their shoulders more perpendicular to the direction of the cast at the start of the forward cast.
The former style give a longer casting stroke but makes tracking and hauling more difficult. The latter shortens the casting stroke, but makes tracking and hauling easier.
Casting in a 45 degree plane, how I fish for instance, is not so good for distance. Tracking and accuracy are much more difficult and you don't get aerodynamic lift even from a wedge shaped loop.
Lastly, the best conditions for distance are very hot days in a low pressure system with near 100% relative humidity (Avagadro's Law and the atomic weights of hydrogen, oxygen and nitrogen). No, I really don't care much about distance practice or results.
Check out sexyloops website for distance casting and do a few searches. Most of those guys are obsessed with distance casting.
It doesn't take much to screw up a distance cast.
Best of luck, and have fun.