As I start to ride longer distances, I wanted to see if there is a better more efficient way to peddle. I am good for about 30km but then get my legs get tired. It could just be that I need to get in better shape since I've only been cycling for 2 weeks.

I have read up on spinning vs mashing and realise that I am a masher. I have strong legs so using the lower gears is not an issue for me but that also means I find myself getting tired. I tried spinning but quickly realised I didn't have the cardio to keep it up and that I can go further mashing.

What do you do and any advice for me?

You basically have 2 options. Interval training (mixing spinning and mashing) or the step-mash method. Interval training works well, because it will help you build cardio and gives your legs a break from just solid mashing. Or, what I mean by step-mash is you build your endurance over a period of time by adding small steps of increased distances until you have reached your goal (time/distance).

What gears do you have?
IF you have a wide range cassette such as an 11-3x and you aren't doing huge hills, change to something like a 12-25. This spaces your gears close and allows you to have a more efficient gear to SPIN.
You don't have to spin like a whirling dervish. Just spin faster and mash less.

there is the basic touring, 'staying on top of your gear' .. find a ratio that you dont have to do either ,
spin so fast, or mash so hard .. press the pedals with a reasonable effort, lightly
and cadence not so fast you cannot keep up with it.

and let the rate of speed be what it will be.. Que Sera.. Sera ..

only really have to work hard when the hill is steep and you are using your lowest gear.

Cycling is a cardiovascular activity It is not for muscle building. If you just want to go further distances you're really not getting a great workout and that's why your body is becoming fatigued You need to maintain a high cadence. You need to be spinning and get the cardio up. You should still be able to talk but not sing Go for short distances and get the cardio up Going long distances will do nothing for you. Spin the pedals fast, It may seem like you're pedaling so fast but going nowhere, it's OK because you'll get better

As i started on June 15th this year i wondered the same but i knew from before to keep my cadence up. So i started slow... 3-5 miles with cadence way up, seemed like i was going nowhere but my knees felt good, i searched for the gear that made me go fast enough to go and the resistance on the pedal was good and fast...This morning coming back home it was a head wind and it tested me, 5 miles in a head wind and the cadence was way up, i felt like i had worked out when i got home, i am not rtying to become a bike athlete but i want to be able to to a 50 mule ride if i want... i am getting there...48 years old and arthritis took a toll on my joints... Mashing is for Potatoes...LOL....

Thanks everyone for the great input. I now know that I need to work on my cardio and cadence.

If you had a cadence counter, it would be a lot easier.

What are you spinning comfortably NOW?

Raise it 5 to 10 RPM.

When that gets comfortable, raise it again. When you can't raise it anymore, give it awhile, and raise it again.

You will be at 80-90, in no time. Then you will wonder how you ever didn't do it, because it is sooooooooo much easier.

Thank you. I guess I need to get riding