I live in the Twin Cities and finding hills is hard. Finding an extended downhill run is even more difficult. In December I had the luck of spending a month in Hawaii and I was treated to only hills.
This has lead me to find sustained hills in Minneapolis and St. Paul. I create this route, 5k to Pike Island. You can see it on my Strava account and you can even export it to your GoRun account and use the turn by turn directions.
Start at the parking area near the John H Stevens house near Minnehaha falls. If you are not from the Twin Cities you will find lots fun names here. Run south on the trail towards Pike Island. Once you get to the dog park you will experience 2 miles of sustained downhill!
Once you are the bottom you have a couple of choices.
1. There is a path up to Fort Snelling, then you can take the road back to the start
2. There is a short cut path that goes up the hill to the frontage road.
The bottom line is that you will end up running 4-6 miles on this route. You can do a hill work out by concentrating on your down portion. Once you get to the bottom you can slow jog back to the top and repeat!