Hi.
If you want to stay around the city, the canal run is probably the best bet, a nice route from the Jury's Inn would be to get on the canal in gas street and then run up to the University and do a lap around here and then run back along the same route:
http://maps.google.co.uk/maps?ll=52.460442,-1.925118&spn=0.008106,0.022552&z=16
The area marked pond is in the Uni grounds and quite nice. That should be about 2 1/2 miles each way. If you want to go a little out of town (only a mile or so) then Canon Hill Park and Highbury Park are good places to run, here's one of my routes:
http://runkeeper.com/routes/rossriley/view/4952
That's a 4 mile loop but add on the distance out to Canon Hill and back and you're probably looking at 6+.
Don't know of any decent shops in the city, there's a small one on Temple Street I think:
http://maps.google.co.uk/maps?f=q&source=s_q&hl=en&geocode=&q=runners&sll=52.462792,-1.915018&sspn=0.064845,0.180416&ie=UTF8&hq=runners&hnear=&ll=52.479477,-1.89935&spn=0.001013,0.002819&z=19
But it's a dual running/cycling shop so I don't know how much stuff they have.
R