You only need to render the areas under the red line that appears in the timeline. This is often quicker than rendering the whole project.
To render an area only slide the large markers at the top to cover just that area and press enter.
Once you have rendered an area you can shorten it without rerendering but if you disturb any effects within the rendered area or lengthen it you have to re-render it.
If everything has a red line to start with then your initial video presets are wrong.
You can sometimes get a better picture of an unrendered section without rendering by selecting High Quality preview although Lower Quality can be faster to show.
Make the timeline as simple as possible by showing only thumbnails of the frames at the start and end of each clip or even switching thumbnnails off except if you really need them. This makes it quicker to zoom in and find a spot in the preview window rather than relying on the timeline thumbnail which only gives you a very approximate guide.
Keep the preview window as tiny as possible and only magnify it when you really need to by dragging it's edges.
I often use the audio waveform as an indicator of where to cut a track to shorten it etc.
If you are going to say have a lot of scenes with a an effect like a dissolve between each clip it is quicker to edit the scenes for length then apply all the dissolves to each cut at the same time afterwards.
It can be easier to manage timing by having clips on alternate tracks. Often less frustrating if you have a common audio to it all and you can have the audio of one track running behind the video of another like a interviewee's face appearing before the interviewer's question has finished. - like this
Even after Rendering, you still have full editing capabilities of that/those Clip(s). When you make a change, the red line will appear in the areas of the change, but you are not limited in any way.
For more discussion on Rendering, and what it does, see this ARTICLE.
One thing to remember is that Rendering is just for Playback. The Rendered AV files will be what one is seeing at playback, and they are linked to the Clips on the Timeline.
It might also be useful to consider what is in the PREL (Project file), to better understand what you see on your Timeline. This ARTICLE has discussion on that, plus, as a "bonus," also discusses the types of Save operations.
Good luck, and hope that this helps,