I recently started doing some longer training rides (1 - 3 hours) and find that I am getting really saddle sore and my arms feel quite uncomfortable. I have a pretty TT riding position which feels great for shorter distances, but it isn't working for the longer rides. I have tried a more upright position and that feels even worse. I am also getting lots tingling in my feet and a bit in my hands. I have tried padded shorts, but they didn't really do anything.

I feel like I want to be have an even bigger distance between the saddle and bars, normally sitting towards the back of the saddle, but when I move the saddle back, it starts getting really uncomfortable.

I could go here: https://www.cyclefit.co.uk/ and spend £200 for a customised fitting session and custom shoe inserts, but I am wondering if anyone has some good tips on what to do to get a good fit. If I change something at the moment, it pretty much just moves the discomfort around different parts of my body.

sounds like your S.O.L

NEVER EVER adjust saddle position to move your body in relation to the bars. Get a longer stem.

Try the following too.
-better saddle
Cut outs are good, firm is good.
-raise bars
give it a few hundred miles
-Gloves and real bartape
-better shoes/pedals
stiffer, possibly with an insole. If you aren't ready for clipless get platforms. Most of the pedals people use on their fixed gears were never meant to ridden in street shoes anyway.

stuff that may work: Bigger frame, Selle Italia SLR, Carbon Fork, wider bars, Specialized bg gloves/shoes

the competitive cyclist fit calculator is generally meant to dial in which frame size to buy, but may also be helpful for you to determine apprx fit measurements:

https://www.competitivecyclist.com/za...LCULATOR_INTRO

you may also want to try to straighten your back a bit by rolling your hips forward, as well as leveling your saddle so you don't put too much pressure on your hands.

stiffer shoes are also really good.

and dutret is right, whenever you make changes, give it some time before deciding it does or does not work

I don't think an expensive fitting session is necessary although I bet it'd work. A lot of people get fit dialed in pretty well by trial and error.

Your fit on this bike does look uncomfortable! See how your arms are stretched out straight? That indicates right away that you might benefit from a shorter or higher reach to the bars. **** man, there's like a foot of drop between the seat and bars---no wonder youre hurting. As well it looks to me like you might benefit from lowering your saddle an inch or three. Maybe you should get a normal frame for long rides and keep the ridonkulous foldie for shorter city rides.

stiffer shoes and or insoles.
gloves.
raise bars and get drops.