C25K advice please

Decreasing your speed is fine. The idea off the program is to do it at a pace you can maintain, regardless of how fast or slow.
Whether you need to repeat or not is up to you. I always suggest trying the next workout, and judging from there. I coached a learn to run program (same thing) and every week my runners would tell me they couldn't do the new workout. Every time, they did it.
If you do find you are really struggling, by all means, repeat. That is the beauty of the program, that you can tailor it to suit you.

Shin splints - if you are on pavement, try going on a softer route, even a track. Ibuprofen AFTER running (not before, not good for you) and ice will help relieve it as well.

I am on week 3 myself - one more run to go. I think you just need to decrease your speed. The first week and a half I kept trying to increase my pace, but then I realized the main point of this program is to increase endurance - speed will come later. Wait til you can run for 30 min, and then start increasing speed little by little. I definitely decreased my speed for the first week 3 run, but then for the second run my pace was about 40 sec faster.

Also, if you are getting shin splints, I also think that means you need to run slower! Your body needs time to get used to this.

Good luck!

I went as far as the end of week 5, but not on consecutive weeks (I did other exercise in between).

I focused on the time I had to jog for, not the speed. The trick for me was just to keep jogging, even very slowly, and not stop until it says. I was surprised how well that prepared me for the next step, even when I didn't feel ready. When I did the third one of week 5, which was a 20min non-stop jog, I think I only did around 1.25 km in the whole 20minutes! That's very slow, but you know what, I didn't care, I didn't stop for 20 minutes!

What I found helped me progess with the programme without fear, was to do extra on occasions. I don't mean run for longer in one stretch, but add in one more cycle of jog walk. I was terrified for the 20min in a row stretch - I mean until the, the most I'd done was 8mins in a row, and not even totalling 20mins jogging in one session, so why did it think I could go to 20 mins non stop? To combat that, the session before, I did 3 x 8min jogs instead of 2. That meant I knew I could at least jog for longer than 20 mins with a small break and felt more confident.

The reason I "failed" at week 5 day 3? well I have very tight leg muscles, and really need to stretch and have a hot bath after I jog. This time I didn't, and the next day I couldn't walk because my back hurt from the tightness in my legs. This put me off. But this week, I'm back, and started again at week 5 day 1 (which I'll repeat a few times before I continue).

I would go slower if I were you. The calf cramps are possibly down to salt and/or water. Try not to drink too much water while you jog or you will lower your salt levels and cramping can happen. A banana half an hour before you go out can really help. Otherwise, if you do need to drink while you're out, find yourself a good ion balanced drink, that isn't high in sugar (I like powerade Ion4). You shouldn't really need to drink in the time you're out though.

For shin splints, slow down, and ease off. You may need to take more rest days in between your jogs. Don't push it with shin splints. You may need to take 1 or 2 steps back in terms of time or speed to let them recover. It's a sign you're probably pushing it a bit too hard too quickly.

I think the trick to following the programme is not to be afraid of going slowly, or of repeating a week or a session, or taking longer gaps for other cardio in between. Don't think it has to be a 9 week programme. Better to go at your pace than to do what I did and quit as a result of something avoidable!

As everyone has said get the distance first then when you can do 5k build on your speed

As for shin splints you tube is your friend for stretches etc