Try to allow yourself some time in Inverness to buy some fuel. There are outdoor shops within walking distance of the bus station. And likely other challengers waiting for the same bus!

I did that journey last year - though getting off a stop earlier at Shiel Bridge.
It’s very easy airport bus to Inverness then the 917 - it’s easiest to book on the city link website.
If you’re getting the evening bus (17.35) it was very full when I used it so pre-booking a ticket would be a good plan.

I’m starting from Dornie this year, but not until Saturday - i’ll Follow your trail.