Firstly, welcome. Ex-Army myself (30 years) and we have had property in Spain for nigh on 15 years and have lived here permanently for the last 4.
Secondly, how well do you know Spain? It is a big country and, as you've said you'd like to be near the beach, the Costa stretch all the way from Gibraltar and around and up past Barcelona up to the French borders so there is a huge area to choose from.
Would you be renting or buying? I would suggest renting to start with. Then you can move around easily if the place you picked turned out to be deserted off season or is noisier than you want or they build multi-storey carparks in front of you or any of a thousand things. If buying, there are one bedroom flats going around here on the Costa Blanca for around €30,000 or less but, of course, they are not right on the beach, mainly 15 to 20 minute drive away. Property is very cheap here at the moment and 3 bed houses are going for around the €100,000 mark or cheaper but, beware, taxes are very high and the transfer tax is now 10% depending on where you buy (Spain is many different "countries" which set their own rates for taxes and so on so look into that carefully. Buying in this area would add a minimum of 15% on the actual buying cost).
A big problem would be healthcare. At 57, you would not be entitled free access to the Spanish NHS so, if you have problems, you would need private healthcare which can be expensive. You will only get the NHS once you have reached pensionable age in your own country. You can, of course, live here for less than 6 months a year and retain residency in UK but anything over 6 months you become liable to officially sign on as an EU foreigner living in Spain which means foregoing UK residency. You would also be liable for Spanish income tax but, as you have an Army pension, it wouldn't attract any tax as it is already taxed in UK.
There are many Brits working over here. mainly in the service industry which is quite poorly paid (about 4 euro an hour) and getting any other type of job would be difficult unless you speak fluent Spanish. So saying, the vast majority of Brits living in Spain are not old and crusty pensioners (like I've just become) and many have a good life over here.
Cost of living is cheaper (except electricity which will be about the same as you pay for electric/gas in UK) but water rates, council tax, car tax and so on are much cheaper.
Don't be put off. We love it here. We live on my Army pension and, with the current exchange rate, it gives us a good standard of living. Unlike just 5 or 6 years ago, virtually everything is available here and Spain is (slowly) coming into the 20th Century and will probably come into the 21st in a decade.
Look around, read some of the comments on other threads here in the Moving to Spain and Living in Spain categories. There is some good advice on there but you'll also get a lot of comments from those for whom it's all gone wrong. Too hot, too cold, missing family, Brit communities (or, as they like to whimsically call them - ghettoes) and not exactly the all year round holiday they were looking forward to.
Best of luck and I can assure you there are thousands and thousands of us who have a great life here.