Dear Joelle, dear Donald,
In my opinion the main limitation seems to be the single condition with a very long trial duration (2 min). Block predictors are already a very crude approximation for longer durations, and in case of a learning paradigm I would especially be surprised if neural activation is constant within a 2 min trial but (possibly) jumps from the 1st to 2nd trial and so on. Another critical aspect is the high-pass filter, as due to the length, there's probably quite some signal within the low frequencies. Put it simply, the paradigm consists of 10 cycles of ~2 1/4 min each, and a default HPF setting will probably remove major parts of e.g. a linear change over time. I don't think adding these 5 min of rest will help. As the paradigm is certainly rather uncommon it would be necessary to reconsider the current predictors, e.g. what does happen within the 2 min, are there any subtrials / components.
Concerning overall length, some labs indeed prefer going with short runs (e.g. < 11 min) but usually, it shouldn't be that much of a problem. In the lab I used to work it was not uncommon to go with a single fMRI run exceeding 20 min, which was conducted successfully with different paradigms and tasks. The data sets I'm currently analyzing are also based on single fMRI runs lasting around 20 - 35 min. With several conditions (say 4 plus a dummy condition for "no-stimulus trials"/blank periods) and in case you're interested in reaction time effects you might want to go with a larger no. of trials than usual (e.g. 60), and with 300 short trials and an average ITI of 5 s this results in a session length of 25 min. In this particular case you could also split the session into several shorter ones, but depending on purpose a single session might be favourable (e.g. if you want to look at changes over time/with trial no. within a single session it's easy to test for linear, quadratic, cubic, ... effects, with separate runs it becomes more complicated as you would have to look at differences between runs but also changes within the runs).
With several short runs it's easier to discard bad runs (e.g. those with large/lots of head motion, those affected by technical failures). In my experience those people prone to head motion will be restless right from the beginning (in healthy students hardly ever an issue, but more likely in patients/other special populations) or start to move after some time within the scanner though, independent of whether it's a single continuous session or several shorter ones. In the latter case you can instruct subjects to remain stable and check fixation of course.