Hi there,
when Mr SG did Edinburgh marathon this year we stayed at the Blue Rainbow Aparthotel Royal Garden, York Buildings Queen Street, Edinburgh EH2 1HY - booked through Expedia.
We paid £227.99 for 2 nights and it was about a 5min walk to his start of the marathon (one of the quick ones, not sure how far it would be to the others).
We were (mistakenly!) given a breakfast basket - loaf of bread, weetabix, jams etc, and the apartment was v good - big lounge/kitchen with satellite TV, dishwasher, free access to washing machine and tumble dryer, 2 bedrooms and good bathroom. Would recommend. Hope that helps!