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Running downhill seems easy because...well...it's downhill. However, you have to have just as solid of a training plan for downhill running as you do uphill running, because it can be extremely taxing on your quads, hamstrings, and shoulders.
I know this from experience! I ran the Mountains 2 Beach Marathon which has a fast, net-downhill course. While I was able to net a PR thanks to the fast course, I had an extremely long recovery compared to other races because my quads were not prepared to handle the extra load and force of downhill running. For the next week, rising from chairs, and (sadly) toilets, was excruciating.
Don't do what I did! Have a training plan that includes downhill running. Ensure that some of your long runs include a downhill section to mimic race conditions. While running downhill, make sure that you lengthen your stride and arm swing to promote good running form.
This article covers comprehensively how to train for downhill running. For trails, this article has more specific techniques, but many of those techniques, like "letting it flow" are plenty appropriate for road running, too.
With time and practice, you'll be able to bomb downhill comfortably, ready to conserve your energy for the uphills. Happy running!