I've only been to the IoM once - arrived on a Friday afternoon and followed the course from Douglas to Ramsey, got drunk, went to Jurby air-field the next day to Marshall a race, got drunk again and then finished off the course on the Sunday morning (in thick, thick fog) to get back to the ferry, so my only lap of the course took me 36 hours to complete = average speed of 1mph
While I may be a little off the pace of the top riders I've got absolute respect for them. I didn't see many bits of the course where I'd be happy hitting 130mph, never mind averaging that over the whole course.