so, I did a little research, and non-stop, from Whiterun to Riften, travelling at moderate speed, without stopping to fight any bandits/monsters/animals along the way, it's about a days travel. But I think, realistically, with stopping to make camps, and weather hazards, and seeing as some of the group are smaller, it would make sense to extend the trip to three to four days. Especially considering the group will be moving cautiously.