Is my goal realistic?
The hardest question in training is not how to build a plan, but whether there is enough time. Here is how to judge it honestly, without a spreadsheet.
Two numbers decide it
Feasibility comes down to how far your goal is beyond your current fitness, and how many weeks you have. If your longest recent outing is already a decent fraction of the goal, you need less time. If the gap is large, you need more, and no amount of enthusiasm shrinks it.
The honest rule of thumb
Your long session can grow only so fast, about 10% a week at most. Count the weeks it takes to climb from where you are now to roughly 78% of the goal at that rate, add a third again for the easy weeks, and add a week or two for a taper. That total is the least time you should give it.
weeks needed ≈ ceil( grow-from-now-to-78%-of-goal ) × 4/3 + taperIf the date is too soon, that is useful information. A nearer, smaller goal now, with the big one later, beats an injury.
Let the calculator do the arithmetic
The plan calculator works this out from your goal, your longest recent outing and your date, and if the date is too tight it offers the earliest realistic one. It is the same feasibility check the app runs before it builds you a plan.
Common questions
How many weeks do I need?
It depends on the gap between your goal and your current longest outing. The calculator gives you a specific number; the rule of thumb above gets you close.
Can I just do the full distance in training?
Better not to. Peaking at about 78% of the goal and tapering leaves you fresher and less injured than grinding out the whole thing beforehand.
What if my date is too soon?
Pick a nearer, smaller goal now and the bigger one later, or move the date. Skipping steps to fit a short timeline mostly raises risk.
Check your own goal in the plan calculator.