Ashtanga is great for building strength in the arms, wrists, legs and core. Because of the repetition of the Surya Namaskar (Sun Salutations) and the constant Vinyasas, the whole practice is a dynamic, flowing sequence that makes you stronger as well as more flexible. There is a lot of weight bearing in the Primary Series. This improves your bone density as well as making your muscles stronger. We also engage the core muscles pretty much throughout. In Ashtanga we call this core work Bandhas.
With the Sun Salutations and the standing sequence the legs become very strong. This is essential for eventually supporting you in more advanced postures but also for helping you improve your alignment and mobility. We work with the legs drawing the muscles upwards and inwards. When we work the legs in this way we also engage the core.
Ashtanga is great for a regular practice
Because in Ashtanga we are given a set sequence it can really help you get into a regular practice. The discipline of the sequence makes you mentally strong as you are able to focus better. The practice of the breath also helps with this. If your mind is more focused you are more likely to practice and your practice will be more beneficial.
Ashtanga is a strong practice so it's always best to start off gently. Do the modifications until your arms get stronger. Keep the lower abs drawn in to support your lower back. Stack your joints and hug the arm and leg bones in towards your centre. As a practice it will get you strong quickly, especially if you do it 3 to 4 times a week. Even twice a week you will notice a difference.
Benefits of Ashtanga
Your stamina and energy will improve as your muscles get stronger and leaner. Your ability to breathe more deeply will improve as the diaphragm gets stronger and your lung capacity expands. Do not underestimate the power of this wonderful practice. It will waken your power both physically and mentally.
I've been practising Ashtanga now for over 16 years and aside from other styles of yoga, I don't really do anything else regularly. I'm always amazed at how much energy and stamina I have when I do something very physically active.
One step at a time, have faith in yourself and enjoy it.