When it procs, it adds an effect that lasts 120s. After shapeshifting, the effect will persist for 120s; then you need to drop back to human to use it.
That said, if you can afford numina and miracle, you probably don't need the Fitness pool - I have my Essence Boost, for example, slotted with 4x Heal/Rech IOs, to soft cap both aspects, and then Miracle and Numina +r/+r in those slots.
Conveniently, Essence Boost (mine is perma) and the +recovery aspects of those IOs, are both 120 seconds.
(The Warshade alternative might be Stygian Circle, but I haven't even bothered with those IOs on my Warshade - perma eclipse and stygian on a super-short recharge means I'm never wanting for endurance)
If slotted in health, will the proc effect carry over into the forms, even if the benefits of health itself do not?