Ah, in a week it really would be pretty straightforward (600k = 100k/day (so 60mi) & a day spare). Might need a bit of route-juggling or some longer/shorter days to make the stops work.
3 days would be strenuous & definitely a cycling-thing rather than a holiday-with-bike. (up to YH at Dolgellau day 1, Dolgellau - Menai & back day 2, back to Chepstow day 3. About 200k/day, bit more on the first & third days which would be ideal as then one could spend a couple of hrs on Anglesey :) ).
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3 days would be strenuous & definitely a cycling-thing rather than a holiday-with-bike. (up to YH at Dolgellau day 1, Dolgellau - Menai & back day 2, back to Chepstow day 3. About 200k/day, bit more on the first & third days which would be ideal as then one could spend a couple of hrs on Anglesey :) ).