Merrien was the first British athlete to finish, coming home eighth in two hours, 20 minutes and 14 seconds.
He said: "The body works harder at altitude and that increases red blood cells which help with endurance events.
"I'm really pleased as I was wanting to get top 20, so to get in the top ten was far better than I anticipated."
Switzerland's Viktor Rothlin won the race, clocking 2'15:31 seconds to finish more than two minutes ahead of Spain's Juan Martinez.
Sunday's race was only the second time Merrien has competed over the marathon distance, with his first outing at the London marathon in April.
And the Sarnian, who usually competes in the 5,000m and 10,000m, told BBC Guernsey it may be a while before he runs the 26.2 miles again.
He said: "I'm just going to have a little bit of time off and I won't run another marathon until next year in the spring.
"I'll be doing the Commonwealth games for Guernsey in Delhi but I won't run the marathon there."
ShareThis