Not locking knees

Standing should be a relaxed, well-stacked position that does not require a lot of tension to sustain. However, if you are used to locking the knees, then when you first become conscious of and try unlocking them, it may be that the legs (often quadriceps, especially right above the knees) feel like they are working. They are just adapting to being used differently -  it is important to have enough muscle tone so that in standing the muscles can support the bones and joints. So the legs getting a bit tired is just part of the transition. Pretty soon you'll develop more tone as your baseline.

The back feeling tired quickly - it's a little bit difficult for me to guess what is happening just from that description. Can you be a bit more specific? Lower back/upper back? And what kind of tired - the muscles working too hard? Are you sure that you are not swayed? And that you are anteverting the pelvis in a relaxed, natural way (easiest way to get there - go into the little squat and then press up through the heels to come out of it)? You never should try to "stick" the bottom out when standing - this would definitely lead to tight muscles in the low back.

Good, I'm glad you figured that out. This is something I really try to emphasize in my classes: the pelvic anteversion does NOT come from forcibly "sticking" the bottom out - that usually results in a sway and tense muscles in the low back.

Getting the bottom angled back from the right place (i.e., the correct pelvic anteversion) is more subtle than that, and for many people, requires re-establishing a bit more flexibility at L5-S1, which we begin doing in Stacksitting and Stretchlying on the side.