I do this route in one go annually, ahead of the runners in the Great Glen ultra marathon with an 0100 start time in early July. It usually takes me around 9 hours for the whole thing, including stopping to chat at the event check points along the way. I love the experience every time; got hosed on for the first couple of hours this year but still enjoyed it all. GPS says 72 miles and 7900′ of ascent for the classic, high route.
As others say, cool your jets early on and be ready for the chunky climbing later. It’s almost all rideable, with just a handful of short pushes in each of the three later climbs. Best bike: probably a fast xc hardtail. Dropper post is more useful than a big fork.