She looks fit to me & not necessarily underweight. If she is, it's not by much I think, tho some other pics of her from different angles would give a fuller picture. I don't think it necessarily sounds like she's in too hard work at all - you don't say what you do in those hours - perhaps it's mainly walking.
I would, however consider changing her feed, whether or not you decide she does need more weight. What exactly is/in the 10% pellets? Whatever protein level you think she needs, I'd be inclined to feed her a chaff based feed rather than pellets, for more chew time & more long fibre. I would certainly be mixing the pellets with a fair bit of chaff rather than feeding straight, if you continue feeding them. I would avoid grainy(be that pellet or otherwise), starchy ingredients as a rule & especially if the diet is starchy/sugary, I'd be feeding the ration over a minimum of at least 3 meals daily. That will allow her to get more goodness out of it and reduce problems associated with that sort of diet.
She does appear to have a rather chunky chest and less developed hind quarters. Also interested if that's the normal way of standing for her, or she's 'camped under' just in this pic? I'd consider getting a bodyworker to check her out & if she always stands like that, consider her heels may be a bit weak & in need of support/protection too.