They weren't designed for old guys ;- ) .
I have a bit of a soft spot for SMLEs, given that they were our service rifle for about 50 years, through several wars. I first shot one when I was about 13, in school cadets. We had SMLEs and Brens in the school armoury. I think that as open sights go they are among the best on a service rifle. They aren't as good as a well-designed peep such as that on a No 4 though. I have both.
The local range runs an Anzac day competition for .303s each year, which includes distances out to 300 m and various snap and rapid and moving target serials. The unmodified SMLEs run in a different class to the No 4s, though the top shots with the SMLE are usually not far behind those with the No 4. I've shot in both classes, and even now I back myself to hit a Fig. 11 consistently in good light with the SMLE at 300 m.
It was recognised even at the end of the 19th century, before the SMLE was adopted, that a peep was better for precision though, and over the years that the SMLE was used in competition a wide range of peep sights were sold and used, most requiring no modification, or perhaps a little bit of shaving the wood near the butt socket, to fit them. A fair number of these went to war too - there are reports for example of Anzac soldiers bringing them and using them at Gallipoli. Here's a link showing a good number of examples of these:
https://www.leeenfieldresource.com/aperture-sightsI have used such sights. Central, Lane, Motty's and Mues in particular were common here, and are still around. Target peeps were also made for fitment to No 4s of course, as these replaced the SMLE in Britain not only for military use but also competition, and I have a Parker Hale PH5C myself.
What also helps greatly for the older gent, for target shooting, is a peep insert which allows you to stop down the aperture in good light, to enhance depth of field and sharpness - the one I have for the PH5C has 6 different sized apertures, and allows you to turn the rear to select the smallest one that still allows you to see the target according to the available light,. It works brilliantly for this. I've also used one with an iris, like a camera. Alternatively, if you want better results with the as-issued SMLE you could do well to get yourself a Merit Optical attachment for your shooting glasses, which does the same thing
https://www.midwayusa.com/product/1015167397 . This will make it a lot easier to use those open sights - like being a young bloke again!
HTH