Mass Effect's story is long and can be challenging on the higher difficulties. The side quests are not needed for this achievement. You are looking at dozens of hours of time so make sure that you go for other achievements while on each playthrough.
For instance, insanity unlocks after beating it on hardcore so play hardcore first playthrough and insanity the next. Also, you can use the same character from the first playthrough to make it easier for you. One playthrough can be renegade and the other be paragon.
Make sure that you save a lot because you can forget to save and lose forty-five minutes of game time.