flying shorter routes will generally boost your profit margins by a significant amount. [ as a general rule]
There are certain price breaks.
50 miles
100 miles
500 miles
1000 miles
2500 miles
5000 miles
and likely others.
Being just above a price break will make a route quite profitable usually. Being just under will make it tough. But that varies.
Also, shorter routes are generally more competitive, as the barriers to entry are far lower. But that doesn't stop markets like LHR-JFK becoming massively overcrowded.I regularly find short routes priced down to 20 of their original market prices. Whereas longer routes tend to be more about clever manipulation if you know how the system works at all