Plumbing – How to fit a length of drainage PVC pipe between two existent fittings

You've already accepted, but to properly fix this:

I'll assume it's all polyethylene pipe (typically used for well piping). The fitting also looks like nylon, which in my experience is easy to damage and often leaks. I much prefer PVC (which is typically gray) or brass.

Take everything apart, remove the elbow, and throw it away. Heat the end of the poly pipe with a torch, both around the outside, and a tiny bit on the inside. You want it to be warm, but not melted/squishy/burning. Put on two clamps, then put the fitting in, and then tighten down the clamps (about 1/4" apart, give or take). Repeat with the other end.

If you've never done this, you may want to practice on some scrap pipe first, especially if you don't have a lot of extra to work with. If you do melt the pipe, you need to cut the end back and remove the melted piece.

45 degree bends make the pipe easier to snake out if it becomes clogged. This can definitely save you a headache in the future.

Some suggest using a catchbasin with a grate below each downspout rather than connecting the downspouts directly to the underground pipe. This gives one last chance to keep leaves and dirt out of the underground piping.

Slope is definitely important for proper drainage.

As far as distance from the foundation, if you choose to use non-perforated pipe, I suppose you could run them closer to your foundation, but I'd avoid it if possible.