One can say:
No hurry. We have plenty of time.
But this implies having lots of time.
What if I would like to say, that we have just some reserve of time — it's not plenty, so we can't relax and at the same time there is no need to hurry — just keep a regular tempo.
There is a word leeway, but I don't believe it's an answer to my question.
I found another word — sufficient time margin, but still doesn't sound like the best option.
There is a word leeway, but I don't believe it's an answer to my question.
Correct, leeway is not the right word. Leeway is defined as "the amount of freedom to move or act that is available" and is a noun which is often applied to signify a specific movement. For example, the deadline is 15th August but we have 2 days leeway either side.
Found another one — sufficient time margin, but still doesn't sound like the best option.
Agreed, this can be used, but is very formal and sounds like something in a legal document or a report.
No hurry. We have plenty of time.
Does indeed imply that one can sit back and do nothing because there is plenty of time.
I would use phrases like
It would also help to be specific, for example,
Idiomatically we don't say we have just some reserve of time. We say...
We [do] have some time in hand
in hand - ready for use if required; in reserve (definition 22.2, oxforddictionaries)
Note that OP's just doesn't work in such contexts, but including optional some in the expression implies that although there's not much additional time available, it will probably be enough. You could be more explicit with, say, We have a little time in hand, but in practice some implies the same.
Optional do above adds a degree of emphatic assertion (and/or refutation, if the speaker is countering someone else's opinion that the goal can't be achieved in the available time).
