Since our science is better than in the 30' or 50's, it is no longer possible to even fantasize that Mars and Venus are double earths they way Heinlen or Bradbury could. Venus is not a warm, rainy earth (The Illustrated Man) and there are no ancient Martian cities to explore (The Martian Chronicles).
Space is vast and emtpy and takes huge amounts of resources just to get a little way. In an age of chemical rockets and bound by a speed of light barrier, space it not the source of infinite possibilities like Star Trek made it seem(which is hardly a good example of hard science fiction).
So why limit yourself to chemical rockets??? In Niven's known space universe, nearby stars are settled by people traveling in nuclear powered (fusion) Bussard Ramjets. Kinda slow, but you can get from here to there in less than a lifetime, when relativistic effects are included. The Traveler Filip(sic?) (Baby Elephants in Pink Elevator Shoes!) in Niven and Pournelles "Footfall" come to the Solar System in a similar ship, and we go to defeat them in "old bang bang" a ship powered by exploding nuclear devices. (Project Orion showed it would work, in the 1960s).
Those were real hard science fiction. As was "Lucifer's Hammer". I'm reading a couple of the more recent installments of the Man Kzin war series. With the exception of FTL ships, which did not exist at the beginning of the war, those are hard science fiction as well.
I must admit I'm partial to the Honor Harrington series as well, but recognize that its pretty old formulae stuff, just well written and entertaining. OTOH, that's what I want.
Then there is the Harry Turtledove series that started with the "World at War" series. (I think of them as the Lizard books). Nothing impossible going on in them either, although some improbable things perhaps.