Honestly, I would say keep doing down dog and forward Ben's. The flexibility in your legs will come with time. Focus more on your alignment and use modifications to do that. For example, in DD, raise up on your toes (like you were standing on tiotoe). This allows you to straighten your legs more. Over time your hamstrings and calves well gain flexibility and eventually you will be able to lower down on your heels. Focus on getting your arms and spine in one long line. That is more important than keeping your legs straight and heels down. Another alignment pointer for downdog that I found really helpful is to keep your fingers spread almost as wide as you can with your index pointed straight to the front, focusing on pressing firmly through each finger evenly, and this is the kicker - rotate your arms from the shoulders so that the inside of your elbows (like where you get blood drawn) face forward. This puts your shoulders into a good alignment and let's you get deeper into the pose and really align your arms and back. Also, focus on keeping your head in line with your arms and had. I know when I started I had a tendency to lift my head to look at the mirror or just let it hang low. I know I just talked a lot about your arms, not legs, but when I started yoga I was frustrated with forensic because of how inflexible my legs were. When I learned to focus on alignment of my upper body in that post and stopped focusing on straightening my legs, I really started to like that pose more, and then my legs just slowly got more flexible - and I still remember the first time I got my heels firmly on the floor with straight legs - it was really exciting. It was also about a year after I started practicing - and that's ok. Flexibility can take a long time, and yoga is about so much more than that. Just keep going with it and you'll get there eventually. Happy yoga-ing!