Yes, if it is iterative or progressive should be understood from context.
Они собирались пойти по магазинам. ('They were going to start shopping')in Russian, but how would you translate something like the past progressive, as in 'They were going to go shopping'
or
Они собирались походить по магазинам. ('They were going to spend some time for shopping')
In Russian verbs of motion can be either iterative or progressive. That is we have separate verbs for iterative motion (like "ходить") and progressive one (like "идти").
Они долго ходили по магазинам.or the past perfect progressive as in, 'They had been shopping for a long time'?
Actually, I don't understand why perfect is needed here. It is past and it means that action is over anyway. Why don't you say just "They shopped for a long time?"
У меня будет книга.Also, if you have 'у меня книга' for 'I have a book' how do you express possession in the future tense, as in, 'I will have a book'?
"I have a book." = "У меня [есть] книга." where "есть" can be skipped.
Literally: "A book is at me."