Hi Robin...M-25 is very scenic nice light houses and some good places to stop along the way....most traffic will be in the 60-63mph on good two lane road...

(taken in November just out of Port Austin)
M-13 I travel all the time...its a good road but kinda like small town after small town again 60mph...I know of some real nice back roads in this area if your interested email me ...they won't take you any longer (probably shorter and nicer)this is the road..
....M-23 is the road we take up to St. Ignace every year...the farther north you get the more scenic it gets....you may think about taking M-32 out of Alpena and ride that to M-31 to M-88 to Bellaire....
If you want more detailed route and things to see along the way email me I'll be glad to help you out...