Half marathon in fourteen weeks, will two lifting sessions help my time or just leave my legs flat on Sunday

Stack hard on hard. Put the lift on the same day as your quality run, a few hours after it if you can, and keep your easy days genuinely easy. What kills people is spreading the stress evenly across the week so that every day is medium and nothing ever recovers. Concretely for your build: Tuesday quality run in the morning, lift that evening, Wednesday easy or off, Thursday easy, Saturday second lift if you want one but keep it short, Sunday long run. You will feel worse on Tuesday night and better on Sunday, which is the trade you actually want.

The kind of lifting matters as much as the placement. The work that shows up in running economy is heavy and low rep, plus some plyometric or jump work, not sets of fifteen leg extensions to a burn. Heavy means low reps and long rests, which is far less fatiguing than it sounds because the total volume is small. Two or three sets of three to five on a squat or a trap bar deadlift, some calf work, some hops, and you are out in forty minutes with sore nothing. If you finish a lift feeling smashed during a marathon build, the session was wrong for the goal.

I lifted every Saturday for a whole build because it was the day I had time, and I ruined about nine Sunday long runs in a row before the penny dropped. My long run pace drifted about twenty seconds a kilometre slower than it should have been and I spent weeks convinced I was overtrained rather than just badly scheduled. Moving the lift to Tuesday fixed it in one week. That cost me most of a training block for something that was purely a calendar problem.

For scale: I kept two forty minute sessions through a fourteen week build at roughly your volume, and the honest summary is my legs felt better on hills, my form held together in the last five kilometres, and my time improved by a couple of minutes. I cannot separate how much of that was the lifting and how much was another block of consistent running. What I can say is that nothing got worse once the scheduling was right, and the niggles I usually collect around week ten did not show up.

Two short heavy sessions, on your hard days, dropped to one in the last ten days before the race. Do not start a new lift inside the final three weeks.