Yup, total bike weight and how many accumulative climbing meters per day, plus rider fitness and age, are all factors that you've got to take into account for daily mileage..
For me, my camping gear summerish load is always going to around 40lbs, with water food, on my study tourer. I've done a fair amount of mountainy riding so can judge days estimates for a rough amount of climbing expected. Would try to reduce load weight a bit, but in the end it will be similar to my recent Latin America trips, a bit less but I know it would be cool in Scotland and I'm a skinny old bugger, so need proper clothes. Plus I reckon stores are far between, so would have to carry more food.
potentially, yeah. Villages/towns can be few and far between. If you ever do decide to tour Scotland/UK in general, do not blindly follow NCN (national cycle routes) some are proper roads, others old railway or tow paths, but others are fields, bridleways, and deep sand pits only suitable for fat bikes. That was another lesson we learnt, actual route planning