In reply to JD19081986:
I did it with my partner in September last year. I was moderately bike fit but she hadn't really been doing a lot more than swimming in the last year and we made it with full luggage (tent etc.) without too much fuss. To prepare, we went on a couple of thirty mile cycles in the Northern Fells - it wasn't enough to give her true comfort but was enough to get her to the end!
In the main, the hills you encounter should be easily eaten up by anyone slightly fit and, if you're going South to North, you get to build up in scale a little, so your legs are more in condition by the time you get there. There are only two hills that might truly challenge anyone, before and after Tarbert and, perhaps, the one out of the harbour on Eriskay. They still shouldn't be insurmountable (and, if they are, you can always push).
Because my partner hadn't done much cycling in the last couple of years, we varied between shortish days (around 25-35 miles) and slightly longer days (around 50-60 miles). In this fashion, we made it from Barra to Stornoway in five days (which included a sensible amount of sightseeing too and some of the customary sitting around for ferries - not just sat in the saddle!). Because of the schedule we were on and because we were cycling back via a couple of things we wanted to see, we didn't go to the very top but could easily have got there on the sixth day (we went pretty near but turned it into a bit of a loop for sight-seeing).
On the way up, we were also beset by a strong and persistent north wind (this meant we started two days late due to ferry cancellations) which constantly worked against us. As such, the five and a half days cycling end-to-end should be even more straightforward if the wind is blowing in the 'proper' direction - if, as you say, your Dad's fit he shouldn't have any issue with it beyond the normal buttock-breaking bike stuff...
