The most important thing is to have respect for your body. Only do things that are comfortable, focus on yourself and not on how you are doing compared to others. Yoga/pilates is not a competitive sport. Over time your body will open up and you will progress as your practise develops. You may feel aches the next day but this is only the body responding to using muscles that are probably weak or under used. Over time these aches become less as your general fitness improves and as your body opens up. You may notice you become more aware of your self and your surroundings. You may become calmer, less stressed. There are many benefits to yoga and they are different in each and every one of us. Just enjoy it!