Quality vs. Quantity

Strength and power enhance endurance while endurance training actually inhibits strength and power.  So to get the best bang for your buck, think quality over quantity.  Form should be the focus first!  All of us have a genetic ceiling over which we cannot pass thanks in large part to our parents.  But none of us have the same neuromuscular limitations as we can always become more skilled.  Practice doesn't make perfect--it makes permanent.  So practice perfection.  This skill will enhance endurance by decreasing the energy cost of swimming/cycling/running/whatever.  And if you throw in the right dash of intensity, it facilitates speed by challenging the different energy systems of the body (i.e. ATP/CP or fast glycolytic rather than just the aerobic system) while helping you ingrain proper form into your muscle memory.  After all, it's hard to go fast with bad form!