Does being a recreational endurance athlete have an impact on the dose of cyanocobalamin i should take monthly (have b12 deficiency).

You don't say if you: are B12 deficient due to pernicious anemia, and consequently, take B12 by injection in high oral supplement doses. The ability, or lack of ability, to absorb B12 in the stomach is the most important determinant of dose. Usual supplement dosage is 1-2 mg/day. I could find no evidence that this needs to be increased or modified in the circumstance of endurance training and competition. Monitor level