Is it a cortisone injection? They can take upto 2-3 days to work properly. They are anti-inflammatory so designed to to reduce/remove inflammamtion at the precise problem area. It may resolve the issue by breaking the inflammationpain imflammation cycle but there's no guarantees unfortunealty.
If the problem is caused by biomechanical issues (poor running style/weak and/or tight gluts/stiff or fallen foot arches can all create excessive torsion/tension at the knee when running) it may well flare up again once you start running distances again if you don't sort those out. One injection is generally thought safe but there are studies showing repeated injections to the same spot actually retard tissue healing and can cause breakdown of the tendon.
A good thing about these types of injections is they allow you to be able to do proper strengthening & rehab (could be as simply as gradually building up distance running over time) without it irrating the knee further. Best advice I can give you on here is make sure stretch your quads and gluts! (2/3 times per day, 45 secs at a time)