This is going to sound harsh, but I say it with all compassion - twice a month for 3 days is too much. You make no mention of a partner or children of your own, but even if you don't have either, you deserve to have a life too.
So my practical suggestions - Work out what you do to support them - is it practical or social / emotional. Then speak to your extended family and firm up their support, eg can auntie Joan visit every other Thursday evening for tea, cousin phil drop by for a cuppa each Saturday morning.
Make sure they have any support going into their home, eg once a month cleaner, a gardener to mow the lawn every fortnight. Are they at a stage where they need more formal help from a carer?
Write out this timetable for them so they can visualise it more clearly, include their clubs and regular outings. My parents local Aged UK group were a godesend - tea dances, music group.
You will then need to gird you loins and grit your teeth to take a stance - decide what YOU want to do. So you may decide once a month / every 6 weeks for a weekend, eg Friday evening to Sunday lunchtime. Sit you Mum and Dad down and TELL them what WILL BE happening - this is NOT a negotiation. You need to make it clear
- you will not be leaving your job;
- you will not be moving nearer - any talk of either of these points, you shut it down - "Common on Mum, Ive already told you that's not going to happen" and change the subject. If the pressure continues, leave the room - "I'm not talking about this anymore".
- this visit is flexible to fit around your work and social life. If you have a good relationship with a member of your extended have them there for this talk to show you are not their own support. And when you are at your parents have things to do that involve others so they can see you are not their only source of support.
This may probable feel harsh but you will have to stand up for yourself as your parents have already demonstrated they have no shame in emotionally manipulating you. Yes, they may miss you, feel scared at their aging, but they have plenty of support from others, they do not need you 100% body and soul.
From your describtion your parents sound reasonably fit and well so this is a marathon not a 100m sprint. You need to pace your input to keep you fit and well, both physically, emotionally and socially. What you mustn't do is let any guilt take over your life because it's your life to live, pure and simple.
My apologies if this seem hard and harsh, well actually I'm not. Draw those boundaries sooner rather than later.