The effects of altitude are quite random, and not necessarily related to how fit you are. The trick to avoiding sickness is not to over-do it. The effects of altitude are mostly likely to come on once you’re above 3000 meters, and once you’re at that height it’s not wise to ascend more than 300-400 meters per day. Sometimes that will mean short walking days of only a couple of hours, but if it means you don’t ascend too high, too quickly, this is necessary.