I think this is not all that bad. What you can do is either do some erect stretching (very effective when you want to make flaccid gains into erect gains) or maybe throw in some jelqs (good for girth, but also very effective length exercise). But if I was you- I'd consider this to be a bonus. That way you not only stretch, but you superset different exercises. Once your erection subsides- go back to your normal stretching routine. If you do what I wrote above, not only will you be able to continue your routine, but you will upgrade it to the level where you gain more and combine it with some girth methods giving you even more gains. I think this is something you should be happy and excited about
If you really want that boner out of the picture- masturbate and afterwards do some DLD Blasters principle (tire you PC so that you could get the most effective stretch ever). The reason why DLD Blasters is one of the best (if not the best) length exercises is behind its principle (you tire you PC off, so that it does no "defensive kegels" and not resist your stretch. Once I read DLD saying that is the surgery method- they cut the suspensory part and send people home with
extenders and stertching routines. Doing what I wrote above is a perfect way to mimic this whole thing. Therefore, this would be as effective as having a PE surgery, but with the difference that you PC will be stronger, it will safe and natural, and no one's gonna cut nothing). Try doing both and see which one works better for you.
Another thing is- post masturbation stretch. Refractionary period should be long enough for you to do your lenth session. Plus, it is the best stretc you can do. See
this! (Click th "this" button). Hope I helped. Try this and tell me what works for you