When ascending Mount Damavand, since you will be carrying one days pack (back pack), you wont need to be very fit for the first two days. But the most difficult part will be on the third day of the ascend, which will be from Bargah Sevom to the summit and returning from there.
Please Note: The general time for the ascend from Bargah Sevom towards the summit will be 6-7 hours and returning from the summit to Bargah Sevom 3-4 hours.
For the adventure travelers that have a mid-level of fitness have better chance of reaching the summit.