As regards the yoga tradition you have been following, I would like to suggest you to go for warm up (15 minutes) before performing yoga postures and yoga postures for 40-45 minutes only. This time period would suffice the whole yoga postures tradition.
You feel worn out and muscular fatigue - it indicates some erratic performance either in terms of time period or way of performing, i.e either you are over-straining your muscles or you are performing beyond your capacity. There is very easy way to check as to whether you are on the correct track - during erratic performance, your breathing may get abnormal or pain of over-strain - and at that time, you should stop immediately. If you are feeling comfortable in one posture for a certain duration, you are performing correctly. With in the comfort zone and without temporary excitement, you would be better able to increase your flexibility.
Such type of fatigue also occurs even in the absence of practice of relaxation which is usually done at the end of the performance of yoga postures. Some yoga therapists suggest to do in between also.
You may start from light postures to strenous postures very gradually. This system can be achieved by doing some sets of postures on one day and other sets on alternate day.
Check your weight also as well as carbose level you are taking in every meal. In this context, I would like to suggest to take some glucose-water in the beginning or at the end of yoga session to compensate your glucose expended in exercise session. Also nature of your sleep can cause this type of problem. So, check whether your sleep is sound and deep.
At last, I would like to state that there are certain factors that lead yoga enthusiasts to suffer fatigue. So, without getting your complete case history, I would not be able to give your accurate solution. However, the above points have been mentioned which I hope would be satisfactorily able to help you in sorting out the cause.
