The next year at the Boston Marathon, the doctors on the starting line advised him of his heart murmur and told him that he should drop out if he were fatigued and that he should not run any more races.

In his autobiography, he gave his reasons as (1) continuing warnings from doctors that he was endangering his health, (2) concern that striving for individual athletic glory was incompatible with the spirit of his religion, and (3) demands on his time from the University extension courses that he was taking at Harvard and Boston University.

In June 1915, DeMar received an Associate of Arts degree from Harvard while working as a printer in the Boston area.

Although he ran some races while in the army, DeMar did not return to serious competition until 1922, when he again won the Boston Marathon in a course record 2:18:10.

He won at Boston again the next year to secure a spot in the 1928 Summer Olympics but ran poorly there in cool weather, finishing 27th.

In 1929, DeMar married and took a job teaching printing and industrial history at Keene Normal School to secure a more stable income.