twinkletwits99 there is a lot of good advice on this thread, but also some nonsense. I have a degree of prolapse and started this long-running thread ages ago, which you may find useful to read through:
www.mumsnet.com/Talk/womens_health/3332405-Any-Pelvic-Organ-Prolapse-Surgery-success-stories
To summarise this thread:
You def need to see a pelvic physio once these services are seeing patients again (do not wait till you have finished having children, as a PP suggested). My GP had never offered to refer me and I didn't know they existed for a while. I had to ask for referral. There wasn't a long waiting list in my area.
The physio will assess you internally and check whether any structures are damaged (and whether any prolapses are present), will assess your pelvic muscle strength, will assess whether any part of your pelvic floor is in spasm (and teach you how to stretch/relax it), and will ensure you are doing kegels optimally. They will also assess whether you need referral to a urogynaecologist (for possible surgery).
In the meantime, definitely download the NHS Squeezy app and do 10 x long holds and 10 x short contractions, 4 times a day. It can take a good 3 months to see any improvement. Also make sure you are well-hydrated, and getting enough sleep, as these factors affect pelvic floor comfort. Other factors that affect PF strength are hormones - BFing and menopause will often worsen things 
. It is not recommended to stop BFing, as the worsening is only temporary in this case.
My pelvic physio said that research shows electronic toners and pelvic floor cones to be of no greater benefit than correctly performed kegels, although women on here have had great success with toners, (esp if they have nerve damage in the area).
The fact you have a feeling of heaviness at the end of the day, and tampons falling out, does suggest prolapse. There are different types. Damage to front wall of vagina can cause bladder to bulge into vaginal wall (cystocoele). Damage to back wall allows rectum to bulge (rectocoele). And sometimes the cervix can drop downwards. This video explains prolapse well:
It is true that surgery would not normally be considered before your family is complete. And even then, the choice to have it is quite cost-benefit dependant, as it is not without its risks of causing other problems, or failing after a few years. The type of surgery would depend hugely on what the exact nature of your problem is. To the PP who described it as "cosmetic".... that is absolute rubbish! It is not a vaginal tightening to make you look pretty and increase your partner's pleasure! It is a surgical procedure to treat unacceptable symptoms, to improve a woman's quality of life!
Also, the "being able to fit 2 fingers in" description is absolute nonsense - ignore. Most women over the age of 18 could fit more than 2 fingers in their vagina!
Also, you are right. It is NOT just about whether or not you leak urine either. Pelvic floor damage can cause a variety of unwanted symptoms. And I see a poster has recommended that kegels do not work, and to do squats instead - v bad advice, not backed up by research.
Running is not advisable. Some women do choose to continue but def get assessed by a physio first. Can you switch to cycling whilst we are socially distancing, and consider swimming afterwards? Pilates is great as it improves core strength, but some exercises should be modified for prolapse, so find a class run by a pelvic physio. In the meantime, look up Michelle Kenway, who does prolapse-friendly pilates on Youtube. Improving core strength, and being shown how/when to breathe during exercise, is all beneficial.
Hope all this is helpful.