Hello Webby, 60 to 75 miles is a pretty tall order, finishing at Snowdon makes it even more so, and I expect this is why no-one's come up with any suggestions yet.
I have rummaged through my collection of books and old magazine clippings, and most of the walks are less than half the length you are looking for. The best I can find is a 4 days / 51 miles hike between Conwy and Llanberis. It doesn't include Snowdon, so with a diversion to that summit it would bring the mileage up to closer to what you are looking for.
Very briefly, you start at Conwy (it's not easy describing this because the original route is from Llanberis to Conwy!) from where you make your way westwards then southwards into the Carneddau via Tal y Fan, Foel Lwyd and Drum, passing Llyns Eigiau and Cowlyd. From the southern end of the latter lake, you make a diversion over the peak of Pen Lithrig y Wrach to Carnedd Llewelyn, the second highest peak in Wales.
Returning to the lake, continue southwards past Capel Curig and scale Moel Siabod, from where you pick a route southeastwards to Dolwyddelan. Continue southwards then westwards to Blaenau Ffestiniog, then you need to pass Tanygrisiau and cross the Moelwyns northwestwards passing the Llynau'r Cwn (the Dog Lakes). Reaching the A498 at Pont Bethania, cross it and climb the Watkin Path to Snowdon.
From the summit you have a choice of routes down, either the Llanberis Path which will take you straight down to that town, or the Rhyd-Ddu Path from the end of which you can head northwards to take in the summit of Moel Eilio before descending to Llanberis.
If the above appeals to you, you can pm me and I could send scans of the route as attachments (but there are 5 pages which include photos) or I could send photocopies by snail mail, but you will need to be very quick because I am going away first thing on Sat 5th after which I wouldn't be able to send anything for two weeks.