This happened to me a few times. Extensor tendonitis. Take a few days (1-3) totally off, ice a ton. When you are coming back, start out real easy on a treadmill, preferably in minimal shoes. Try barefoot running too.
The extensor tendons on the tops of your feet are under the most stress when they are working eccentrically to lower your foot from a dorsiflexed position after heelstrike. You definitely injured them running the steep downhill--a downhill slope means more deceleration has to happen upon heelstrike. So, eliminate the heelstrike, eliminate the problem. Barefoot running, and treadmill running (which both reduce the need for the extensor tendons to decelerate your foot slamming down after heelstrike) will let you get back to running speedily. You will probably find faster work (especially in flats, or other shoes with a low heel) less damaging. Also, avoid hard surfaces, as the hardness causes your foot to slam down faster, working the tendons much harder.
Lacing can aggravate the problem. Do "Lydiard lacing," it is great.
Lydiard lacing can help
Anyways, this will heal fast if you let it. Just hit it up with the ice cups 3-5x a day.