A definition of base could be several things.
A chronic training load that is high enough that allows you to do more work of various speeds without becoming overly fatigued.
Or to take base from an endurance standpoint.... endurance is a function of time and effort. The faster you are at the top end the faster your % of effort at any given distance will be. i.e. if your top end says you can swim 1000y in 10 minutes and your buddies top end says he can swim 1000y in 12 minutes then you will be able to swim 2000y faster than your buddy.... so your buddy should work on speed and not spend time working an aerobic system that barely scratches the surface of stressing ones system. Using a maximal lactate steady state baseline time of one hour any effort/speed beyond that one hour is HIGHLY correlated to one's ability to perform work for that one hour. Any discipline shorter than one hour is highly influenced by ones ability to derive energy from their VO2 system. This is why you see many MOP IM athletes not able to race a sprint at speeds any faster than they might race an IM.
Mammals respond to aerobic stress by getting faster (big generalization but it'll do for now).... L1 hardly stresses you at all... hang on... lemme go float.