If the warmup is easy for you but the WOD is challenging, you're in good company!
Since the WOD changes every day, there may be some things some days that will be easy for you and other things that will be hard, depending on your fitness.
If the WOD prescribes something that is difficult for you, this is good news, because it means that now you have a goal. For instance: if a HSPU is a challenge, then your goal can be to complete one HSPU. You can train for your HSPU every day even if it's not in the WOD, because it's a goal you're working on.
To progress up to something, go backwards from the "goal exercise" to its most basic level. For example, with the HSPUs, just go for a handstand. See how long you can hold a steady one. Use a wall for support if you need to. When you can hold a handstand without assistance for several seconds, try working up to a pushup. It may take time, but with consistent practice you'll get there.
If the WOD has things in it that are difficult for you, you can set your goals and on those days instead of the "difficult" (goal) exercise you can work on wherever you are in your progression towards it. If the WOD says 10 HSPUs, try holding your handstand for as many seconds as possible. Or set a timer for two minutes, and do a handstand until you get tired. Record the number of seconds you held it. Rest, and try another handstand when you feel ready. Repeat this for the full two minutes, keeping track of the total time out of the two minutes that you were in a handstand. See if your total sustained time within the two minutes increases over a period of weeks. It can be a good way to track progress.
Ultimately, do the stuff in the WOD you can do; progress up to the stuff that proves to be a goal for you. Ultimately as long as you do something every day that develops strength, stamina, coordination, balance, and flexibility you'll be good. Use your goals as a guide.
Regarding how to start
parkour, the only way to start is to start. Check out the landing and roll tutorials. Practice these in your livingroom or at a park for a while. Then go out to a place that looks architecturally interesting, or to a playground, and climb on stuff. Try to get yourself over a tree stump using just your hands. Practice QM and rolls together. Get a group of people together and play "Hot Lava" on the playground equipment. Or QM tag. Once you feel comfortable moving in this way, start checking out the vault tutorials and just practice, practice, practice.
Trust me, it happens sloooooooowly, but it's worth it.
Good luck!
