We asked a few elite runners who frequently do both.
The 41-year-old broke his own world record.
They've made their name in ultras, but they're leaving the trails behind for one day in Boston.
Racing frequently can be risky, but there are also lots of potential rewards to this approach.
The 89K race is slated for Sunday and will use the “down” route this year.
These runners have pulled off some amazing running feats. Are they superhuman, obsessed or just out of their minds?
Professional runner Michael Wardian is aiming for a Boston PR this year as he remembers the tragedy that unfolded in 2013.
Wardian raced 52.4 miles on Sunday at an average pace of 6:15 per mile.
The 39-year old successfully completed the first half of an epic marathon double.