Fitness levels differ from person to person and is therefore difficult to judge if one is fit for a particular trekking expedition. Just as an indicator, we can say that if you can jog or run 5 KM in half hour or walk continuously for 10 KM (with 3-4 short breaks) and if you can hold your breath for 30-40 seconds you can be a trekker. Having said that, you should consult a medical practitioner to check your fitness levels before joining a trekking expedition.