Nice example of the taking rest principle in Vinyasa Krama. Navasana is a challenging posture that can get the breath and heart rate going, if it does Ramaswami encourages us to take a short rest.
The first posture (pic 2) is salambasana navasana, supported boat pose. If you find full Navasana challenging then you might want to stick with the supported version or do a mixture of the two, a few breaths in Salambasana navasana then an attempt at navasana before returning to a few more breaths in the supported version.
In Navasana let physics do some of the work. Raise your legs but then pivot backwards on your backside as a counterweight to your legs which will bring your legs a little higher.
Stretch through the legs does a better job of straightening the legs than trying to lift from the feet.
The same goes for the back stretch through the back as in so many other postures in Vinyasa Krama, stretching/lifting out of the pelvis, try to flatten the small of the back as in yesterdays table pose.
If like me you have a pronounced coccyx then navasana is particularly challenging, try to avoid settling on the tip of the coccyx. I have to be a little above mine which means I need to place greater emphasis on straightening my back.
For the final posture Urdhwa pachimottanasana you might find it easier to begin with gripping the ankles and finding your balance then as you fold closer to your legs shuffle your hands up you your heels.