Many times around the world travelers who think to realize this salkantay trek fitness level is too easy they're wrong, it's not very difficul either, the age doesn't matter because any age can do it. But is recommended not to bring younger kids or someone with cardio problems, if that was the case is better ask to your doctor for a cardio test. In general is better be in a good shape but normally the hikes are done at a slow pace which is easy to stop for a moment to rest. Also don´t forget that this trip climbs up to 4200m in height. Therefore, we don't recommend people with respiratory, circulatory or cardiac problems or very young children. Is for that I said before is better ask to your doctor and can choose if you can do the tour or not. About the backpack in your resistance is very important too, either many tourists walking with unnecessary equipment and then suffer the consequences, have to know what carry and what no.
For example, a good backpack on hand can be only your additional clothes, good trekking shoes, waterproof (especially in salkantay rainy seasons) where the route is more challenging. The camera, sunglasses and sunscreen can to carry in the hand. Remember the acclimatization and the fitness level are both things differently, a few days in Cusco before the trek could be fine to acclimatize the body also try to purchase the "hoja the coca" because if you suffer of mountain or altitute sickness a coca tea would be excellent to minimize the symptoms, and during the road should chew this hoja de coca to obtain more resistance. Now about the physical condition, there are a couple of factors, for example the food where it will be helpful in your salkantay trek, the beans; lentils and the quinoa are natural ingredients increases your power too much. On the other hand, the correct exercises as running for the mornings or maybe swimming a couple of times per week would be fine and in this way you should be ready for the hike. Now these classic advices that will can help you.
Go in your own ritme, it's important because it not a race it's an excursion and you have to explore and take pictures to the archaeological sites, all that taking your time.
The style of your steps, each one know how walking especially ascending, but is better in this circunstances don't be delayed with the group.
Have good trekking shoes, remember is not only purchase the shoes, if your boot or shoes are news try to use before the trek for softem, and avoid the annoying blisters.
Carry a cream for blisters, maybe can result absurd but is useful when the blisters are already annoying, also bring a first aid kid for some problem you can have.
Additional socks, are light and easy to wear, you can use to warm your fingers during the night or to adjust when your shoes don't fot wel, also if the actual socks get wet.
Walking sticks, is just an alternative, some people like it and some people not; but is recommended because help to ascending and descending easier, is possible purchase in Ollantaytambo this product but maybe more expensive.
The "hoja de coca", it's very important for all the hikes because when you chew it will have an amazing energy and resistance to arrive until Machu Picchu, also when suffer the mountain or altitude sickness just prepare in the night a coca tea and the symptoms will be more calm, it wonderful and magic product can buy in local markets of the area.
Sunglasses and sunscreen, generally everybody bring both, but if you want buy here in Cusco is better go to authorized places or browser the item site.
Eat a good carbohydrate diet, remember consume in every hike at high altitude.