In reply to Eric the Red: I did Kiel marathon.. 4 10k laps around the ferry terminal..
Otherwise I've only done Jungfrau, Loch ness (not that quick some sharp climbs), Snowdonia (4 times), Kiel.
And then in the States Austin, Boston, Fort Worth and this weekend Mt Desert Island.
2:42 (Austin), 2:44 (Boston), 2:46 (FW), 2:50 (Kiel), 2:51 (Snowdonia '11), 2:53 (LN), 2:56 (Snowdonia '10), 3:01 (Snowdonia 09), 3:03 (Snowdonia 08), and 3:31 (Jungfrau).
Snowdonia isn't that much slower than Loch Ness, which is down hill but some short sharp climbs.
Potentially Boston should have been my quickest but it was 30 C, and Austin was ran off zero rest and its a hilly course. 2:51 in Snowdonia is probably my best run though.