I take it as praying as often as you can. Because a prayer helps even if you can’t concentrate on it and it just takes physical effort to pray and there isn’t something there like an emotion or concentration in the first place.
It’s also true that I mix this advise up in my head with “pray unceasingly” which is clearer for me.
I also think about the saints. When we ask them to pray to God for us, we expect them to - pray for us. We don’t expect them to wait and see if they consider our demand and only to pray when they mean it. We just expect them to do it! In all honesty this is the tone of all of the prayers dedicated to them. So why not pray often even if we don’t feel it? That is my argument for “pray hard”= “pray often”